Is it permissible to boycott relatives for agreeing to marry off one of their girls to a young man whose money is illicit? And what should the young man do if he repents? Are the relatives sinful? And what advice do you offer the girl?

The young man who earns unlawful money should be advised gently and kindly to repent, and that he must get rid of the unlawful money by spending it on charitable causes. He is not permitted to benefit from it himself except to the extent of his need if he is poor. If he repents, then praise be to Allah. Otherwise, he should not be accepted as a husband because he is not pleasing in religion and character. The girl's family and the girl herself should be advised by explaining the harms of marrying a dissolute person. As for shunning them, it is not permissible except for a legitimate reason, and what you mentioned about fearing the mixing of your lineage with theirs is an undue strictness. Maintaining kinship ties may have a good effect in giving advice and promoting reform.