first lodge a general diary over speed post, mention all facts without hesitation and submit the same.
get it written by her, that she does not want to stay with the your brother and his family and do get it written over email or whatsapp, there is a higher court judgement which says, if a hindu son is separated from his aged ailing parents then it is a very good ground for divorce, but to file the same on the same ground, you need to have strict evidence, so get your proof ready and then file for divorce.