Today, I have gone through your earlier posting of 2 to 3 months back. Now, you must have analyzed that whose advises turned out to be true and whose were not.
First, I deal with legal aspect. The child is six months old and hence, in all probability, your sister will get the custody of the child. You said your sister is a working woman. If that is so, and your BIL also a working person, then the custody goes in your sister's favour. Normally, till five years, the custody will be with the mother, due to express provision in law. Even after that, the father will be generally given visitation rights and custody will be given to the mother. If he is interested for the 2nd marriage, he will not insist for custody or visistation rights.
Your inlaws cannot forciably keep the child along with them. your sister willl get the custody.
If BIL goes to second marriage, without getting divorce, he will directly go to jail under bigamy offence. Your sister also will get the divorce on the ground of his adultery.
If you put allegations on them that they are cruel and again insist that you want to save your sister's married life, that is not possible for the reason that law can punish the people but cannot reform them. So if your sister wants to protect her matrimonial life, she has to bear their cruel behaviour till her last day, as those people will not reform. If she does not want to continue with him, she can go for divorce and contest the child custody case, if they file a case for child custody.
