She committed incest with her brother – should she tell her husband?

What you have to do is to repent to Allaah and seek His forgiveness for what you have done. What you have done is no minor matter. Allaah says (interpretation of the meaning): 

“And come not near to unlawful sex. Verily, it is a Faahishah (i.e. anything that transgresses its limits: a great sin, and an evil way that leads one to hell unless Allaah Forgives him)”

[al-Isra’ 17:32]

What you and your brother did is a kind of approaching zina (unlawful sex). Allaah says (interpretation of the meaning): 

“And those who invoke not any other ilaah (god) along with Allaah, nor kill such person as Allaah has forbidden, except for just cause, nor commit illegal sexual intercourse and whoever does this shall receive the punishment.

The torment will be doubled to him on the Day of Resurrection, and he will abide therein in disgrace;

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 25:68]

And the Prophet (peace and blessings of Allaah be upon him) said: “O ummah of Muhammad, by Allaah there is none more indignant than Allaah when His slave, male or female, commits zina. O ummah of Muhammad, if you knew what I know, you would laugh little and weep much.” 

Narrated by al-Bukhaari, 1044; Muslim, 901. 

So what you and your brother have to do is to repent to Allaah from what you have done, and to do a lot of righteous deeds such as praying, fasting and giving charity, so as to atone for your evil actions. 

What happened between you and that man is another sin. Our advice to you is to check yourself and realize that when Allaah becomes angry with His slave He dooms him and removes blessing from his entire life, if he does not repent and turn back to his Lord. 

With regard to telling your husband about what has happened to you in the past, you do not have to do that once you have repented from it. You should cover yourself and not speak openly of something that you have done in the past. The Prophet (peace and blessings of Allaah be upon him) said: “All of my ummah will be forgiven except those who commit sin blatantly. It is committing sin blatantly if a man does something at night, then in the morning when Allaah has concealed him, he says, ‘O So and so, I did such and such last night,’ when all night his Lord has concealed him, and he uncovers that which Allaah has concealed.” Narrated by al-Bukhaari, 6069; Muslim, 2990. 

Your parents’ marriage is not regarded as null and void, because there is not connection between what happened and your parents’ marriage.  

“And no bearer of burdens shall bear another’s burden”

[Faatir 35:18]

So if you conceal what you did, and your brother also conceals what he did – which is what you both must do – then the relationship between your children will be very normal and there will be no problems, either from the shar’i or social point of view. 

With regard to repentance from sin and seeking forgiveness for it, there are conditions attached, which are discussed in question no. 13990. 

May Allaah help you to do all that is good.