Should he interfere to prevent the marriage of a girl to a person with whose brother she had a relationship?

Praise be to Allah.

There is nothing wrong with the girl marrying this suitor, subject to  two conditions: 

1.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. 85335. 

2.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.