My sister is married in a sunni family who have got their daughter married in an ahmaddiya family.So, can have good relation with my sisters family or should i abandon her family because they are having relations with ahmaddiyas.

Firstly: 

The Ahmadiyyah – or Qadianiyyah – are a sect that is beyond the pale of Islam. 

In a fatwa of the Standing Committee for Academic Research and Issuing Fatwas (2/220) it says: 

Question: what is the ruling on this new religion and its followers, i.e., a religion that is called Ahmadiyyah, whose missionaries warn people against some verses of the Qur’an or names of Allah, and they forbid sending blessings upon the Prophet (blessings and peace of Allah be upon him)? Where and when did this religion originate? What is the ruling on those who turn away from it? 

alone, and blessings and peace be upon His Messenger and his family and companions… To proceed: 

Answer: The Pakistani government has issued a ruling that this sect is outside of Islam. The Muslim World League in Makkah has also issued a ruling to that effect. The Conference of Islamic Organizations that was held by the League in 1394 AH has published an essay which outlines the principles of this sect, and how and when it originated, and other facts about it. 

To sum up: it is a sect which claims that the Indian Mirza Ghulam Ahmad was a prophet who received revelation, and that no person’s Islam is valid unless he believes in him. He was born in the thirteenth century AH, but Allah, may He be glorified, stated in His holy Book that our Prophet Muhammad (blessings and peace of Allah be upon him) is the seal of the Prophets (i.e., the last Prophet), and the Muslim scholars are unanimously agreed on that. Hence anyone who claims that there is any prophet after him who receives revelation from Allah, may He be glorified and exalted, is a kaafir (disbeliever) because he has disbelieved in the Book of Allah, may He be glorified and exalted, and he has disbelieved in the saheeh hadeeths from the Messenger of Allah (blessings and peace of Allah be upon him) which indicate that he is the Seal of the Prophets, and he has gone against the consensus of the ummah. 

And Allah is the source of strength; may Allah sent blessings and peace upon Allah Prophet Muhammad and his family and companions. 

Standing Committee for Academic Research and Issuing Fatwas. 

‘Abdullah ibn Qa‘ood, ‘Abdullah ibn Ghadyaan, ‘Abd ar-Razzaaq ‘Afeefi, ‘Abd al-‘Azeez ibn Baaz.

End quote. 

Based on that, it is not permissible for a Muslim woman to marry a man from this sect, because it is not permissible for a Muslim woman to marry a disbeliever. 

Allah, may He be exalted, says (interpretation of the meaning):

“And give not (your daughters) in marriage to Al-Mushrikoon till they believe (in Allah Alone)”

[al-Baqarah 2:221]. 

At-Tabari (may Allah have mercy on him) said: 

What Allah, may He be exalted, means in this verse is that He has forbidden believing women to marry a mushrik (polytheist) of any type. So do not, O Muslim men, give them (Muslim women) in marriage to them (mushrikeen), for that is forbidden to you.

End quote from Tafseer at-Tabari, 4/370 

Allah, may He be exalted, says (interpretation of the meaning):

“O you who believe! When believing women come to you as emigrants, examine them, Allah knows best as to their Faith, then if you ascertain that they are true believers, send them not back to the disbelievers, they are not lawful (wives) for the disbelievers nor are the disbelievers lawful (husbands) for them. But give the disbelievers that (amount of money) which they have spent (as their Mahr) to them”

[al-Mumtahinah 60:10]. 

Ibn Katheer (may Allah have mercy on him) said: 

Allah says: “they are not lawful (wives) for the disbelievers nor are the disbelievers lawful (husbands) for them”. This verse is the one which prohibited Muslim women to mushrik men.

End quote. Tafseer al-Qur’an al-‘Azeem, 13/521 

This family whose daughter is married to an Ahmadi man has gone against the ruling of Allah, may He be exalted, and has given their daughter in a marriage that is invalid according to scholarly consensus, and this has made her vulnerable to joining the kaafir sect of her husband, as usually happens. 

For more information, please see fatwa no. 144765 

Secondly: 

What is your duty with regard to this family? 

If this family is unaware of the Islamic ruling on the Ahmadiyyah or is unaware of the prohibition on marriage of a Muslim woman to a kaafir, then what you must do in this case is teach them and alert them to this fact. 

That is because the Prophet (blessings and peace of Allah be upon him) said: “Whoever among you sees an evil action, then let him change it with his hand [by taking action]; if he cannot, then with his tongue [by speaking out]; and if he cannot, then with his heart – and that is the weakest of faith.”

Narrated by Muslim, 78. 

You will attain a great reward for teaching them about the Islamic rulings of which they are unaware. 

The Messenger of Allah (blessings and peace of Allah be upon him) said: “Verily Allah and His angels, and the inhabitants of the heavens and the earth, even the ant in its hole and the fish, invoke blessings upon the one who teaches the people good things.”

Narrated by at-Tirmidhi, 2685; classed as saheeh by al-Albaani in Saheeh Sunan at-Tirmidhi, no. 2685 

But if this family is aware of the ruling on the Ahmadiyyah sect, and knows that it is a kaafir sect that is beyond the pale of Islam, and they know that a marriage between a kaafir man and a Muslim women is haraam, but they did not pay attention to that, then in that case there is nothing wrong with shunning them for the sake of Allah, so as to be harsh with them, denounce the evil deeds, express disavowal of their action and deter them from this way. 

Al-Bukhaari (may Allah have mercy on him) said: 

Chapter on what is permissible of shunning one who has committed sin. 

Ka‘b said, when he lagged behind and did not join the Prophet (blessings and peace of Allah be upon him) – i.e., he failed to join the campaign to Tabook: The Prophet (blessings and peace of Allah be upon him) forbade the Muslims to speak to us… And he mentioned fifty days.

End quote from Fath al-Baari (10/497). 

Al-Haafiz Ibn Hajar (may Allah have mercy on him) said: 

What is meant by this report is to highlight the kind of shunning that is permissible, because the general meaning of the prohibition on shunning a Muslim applies only in the case of one who is not being shunned for a legitimate shar‘i reason. But here the reason for shunning is explained, which is that it is directed at a person who committed a sin, so it is justified for the one who sees him doing that to shun him for that reason, so that he will stop doing it.

End quote from Fath al-Baari, 10/497 

Moreover  there is a greater reason to shun people in such cases, those who commit crimes and sins, if a Muslim fears that if he mixes with them he may be influenced by their sins, or he may join them in that, or that he may be spoken ill of, or that it may be detrimental to his religious commitment or his worldly interests 

Ibn ‘Abd al-Barr (may Allah have mercy on him) said: 

My view is that if a person fears that sitting with him or talking to him may be detrimental to his religious commitment or worldly interests, and may increase enmity and resentment, then shunning him and keeping away from him is better than staying close to him, because that will protect you from slipping and from his specious arguments against what you believe is correct, and you will not be safe from the bad consequences of mixing with them. Shunning (in a peaceful manner) may be better than mixing which may lead to trouble.

End quote from al-Istidhkaar, 26/149-150 

In fact whether you should continue your ties with this family, or denounce them, or shun them, depends on the nature of the relationship between you and them, and what shunning is likely to lead to of serving a legitimate shar‘i interest or may lead to negative consequences that outweigh any shar‘i interest. Moreover we think that what is usual in such cases is that your relationship with them is not strong, and there is no direct connection between you and a family such as this; rather the matter has to do with your sister who is married to one of them, and does not have anything to do with you. 

Shaykh al-Islam Ibn Taymiyyah (may Allah have mercy on him) said: 

Shunning varies according to how strong or weak, and how few or how numerous the people who are doing the shunning are. The purpose is to rebuke and discipline the person being shunned and to deter the masses from doing likewise. 

If the purpose is more likely be to achieved by shunning, and it will weaken and reduce the evil, then it is prescribed, but if the person being shunned and others will not be deterred by that, rather the evil will increase, and the person doing the shunning is weak and the bad consequences will outweigh the good, then shunning is not prescribed, rather softening the hearts of some people is more effective than shunning. 

End quote from Majmoo‘ al-Fataawa, 28/206 

And Allah knows best.