Praise be to Allaah.
There is nothing wrong with the girl marrying this suitor,
subject to two conditions:
That she has repented to Allah
from the haraam relationship, and if it reached the point of zina, it is not
valid to marry a woman who committed zina until after she repents.
See the answer the question no.
That there is no danger of a
resumption of her relationship with his brother. This marriage may make it
easy for the relationship to resume and continue, especially in households
where no attention is paid to preventing free mixing. That would cause a
great deal of trouble, as is obvious.
Based on that, if it is known that she is still in this
relationship or there is the fear that it may resume, based on what is seen
of circumstantial evidence, then in this case the one who has knowledge of
the situation should interfere in order to prevent the marriage, so as to
denounce evil and protect the honour of the Muslim. So he should advise the
suitor to forget about this woman in a way that will not lead to conflict
and division between the two brothers; rather he should speak in general
terms, such as saying; “She is not suitable for you,” or “she is not good
for you,” and so on.
If it is known that the relationship has ended and there is
no fear of it being resumed, then praise be to Allah, and there is nothing
wrong with this marriage, as stated above.
And Allah knows best.