Why is this even an issue for debate. Is your mum-in-law/husband God? Besides it is your husbands chromosomes that decides the sex factor, so focus your mind on something more productive like having a good marriage and let God do his part. Whichever comes first (boy/girl) just be thankful for the gift of motherhood at all. My two cents.

It's simply, report to a well respected and elderly family member, and upon that elder's consent let your sister know what is going on and leave the house. Keeping quite will only encourage the man. Such people usually get desperate, what if he tries to rape you? Please be wise!!!