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.