بسم الله الرحمن الرحيم
The relatives with whom your sister established illegal relationship if he is not among your sister’s father-in-law, father-in-law’s father, her husband’s grandfather, son of the husband etc as it is also obvious from the question then she did not become haram on her husband (your bahnoi). Hence it is lawful for your bahnoi to keep her as his wife and continue marital life with her as before.
قال في رد المحتار (كتاب النكاح، فصل فى المحرمات 4:109 ط مكتبه زكريا ديوبند) عن البحر؛ لا اعتبار لماء الزاني ولذا لو زنت امرأة رجل لم تحرم عليه وجاز له وطؤها عقب الزنا اهـ .
Allah knows Best!
Darul Uloom Deoband