Is it possible to return to this world to do righteous deeds?

Firstly: 

You should note, may Allaah bless you, that people’s life spans are in the hand of Allaah, Who says (interpretation of the meaning): 

“For everything there is a time prescribed”

[al-Ra’d 13:38 – Pickthall] 

“and no person knows in what land he will die”

[Luqmaan 31:34]

It is not possible for anyone to know when he will die; it is only speculation which may be proven true or may be proven false. 

Sickness – for example – does not mean that a person will die soon, just as being young and strong does not mean that a person will live for a long time. 

How often have people assumed that a sick person will die soon, then he has lived for a long time. And how often has death come suddenly to a healthy person. 

The poet spoke the truth when he said:

 Equip yourself with piety, for you do not know, when night comes, whether you will live till dawn.

How many young men laugh in the evening and the morning, when their shrouds have been prepared but they do not realize?

How often do people hope for a long life for young people, then their bodies enter the darkness of the grave.

How many brides have been adorned for their grooms, when it has been decreed on Laylat al-Qadar that their souls are to be taken.

How many healthy people have died for no reason, and how many sick people lived for a long time. 

What is required of the believer is to prepare for death at any time, by repenting to Allaah and doing righteous deeds. Ibn Maajah (4259) narrated that Ibn ‘Umar said: I was with the Messenger of Allaah (peace and blessings of Allaah be upon him) when a man from among the Ansaar came to him and greeted the Prophet (peace and blessings of Allaah be upon him) with salaam, then he said: “O Messenger of Allaah, which of the believers is best?” He said: “The one who is best in attitude.” He said: “Which of the believers is most wise?” He said: “The one who remembers death the most, and is best prepared for what comes after it. Those are the wise.” Al-Iraaqi said: its isnaad is jayyid. It was classed as hasan by al-Albaani in Saheeh Ibn Maajah. 

Who has guided your sister to repent and to regret what she has done in the past. 

We send her the glad tidings that if she is sincere in her repentance, then Allaah will accept her repentance and will turn her bad deeds into good deeds. Allaah says (interpretation of the meaning): 

“Except those who repent and believe (in Islamic Monotheism), and do righteous deeds; for those, Allaah will change their sins into good deeds, and Allaah is Oft‑Forgiving, Most Merciful”

[al-Furqaan 25:70]

See also question no. 14289 

Secondly: 

When the believer enters Paradise, Allaah will give him all that he wishes for, and more than he wishes for. Allaah says (interpretation of the meaning): 

“Trays of gold and cups will be passed round them; (there will be) therein all that inner‑selves could desire, and all that eyes could delight in and you will abide therein forever”

[al-Zukhruf 43:71]

Al-Bukhaari (806) narrated from Abu Hurayrah and Abu Sa’eed al-Khudri (may Allaah be pleased with them) in a lengthy hadeeth that the Prophet (peace and blessings of Allaah be upon him) told them of the last of the people of Paradise to be admitted to Paradise. Allaah will say to him:  

“Tell Me your wishes.” So he will say all that he wishes for, and Allaah will prompt him to mention other wishes and when he cannot wish for any more, Allah will say “You will have that and a similar amount besides.” Abu Sa’eed al-Khudri (may Allaah be pleased with him) said to Abu Hurayrah, The Messenger of Allaah (peace and blessings of Allaah be upon him) said, “Allaah said, ‘You will have that and ten times more.’” Abu Hurayrah said, “I only remember the Messenger of Allaah (peace and blessings of Allaah be upon him) saying, ‘You will have that and a similar amount besides.’” Abu Sa’eed said, “I heard him saying, ‘You will have that and ten times more.’” 

But if a believer wishes to go back to this world then he will not be granted that, because Allaah has decreed that no one will go back to this world. 

Allaah says (interpretation of the meaning): 

“And a ban is laid on every town (population) which We have destroyed that they shall not return (to this world again, nor repent to Us)”

[al-Anbiya’ 21:95]

i.e., the people of the towns that Allaah has destroyed will wish to go back to this world so that they can make up for what they missed. 

See Tafseer al-Sa’di, 868. 

Al-Bukhaari (2817) and Muslim (1877) narrated that Anas ibn Maalik said: The Prophet (peace and blessings of Allaah be upon him) said: “No one who enters Paradise will wish to come back to this world, even if he were to be given all that it contains, except for the martyr, who will wish that he could come back and killed ten times because of the honour that he sees.” 

But that wish will not be granted to him. 

Al-Tirmidhi (3010) and Ibn Maajah (190) narrated that Jaabir ibn ‘Abd-Allaah ibn Haraam said: “The Messenger of Allaah (peace and blessings of Allaah be upon him) met me on the day of Uhud and said: ‘O Jaabir, shall I not tell you what Allaah said to your father?’ I said: ‘Yes, O Messenger of Allaah.’ He said: ‘Allaah has never spoken to anyone except from behind a screen, but He spoke to your father directly and said: “O My slave, ask something from Me and I shall give it to you.” He said, “O Lord, bring me back to life so that I may be killed for Your sake a second time.” The Lord, may He be glorified, said: “I have already decreed that they will not return to life.” He said, “My Lord, then convey (this news) to those whom I have left behind.” Allaah said:

“Think not of those as dead who are killed in the way of Allaah. Nay, they are alive, with their Lord, and they have provision”

[Aal ‘Imraan 3:169].’” 

Classed as saheeh by al-Albaani in Saheeh Ibn Maajah. 

We ask Allaah to bless your sister and all the Muslims with sincere repentance. 

And Allaah knows best.