NO SAH…PLEASE READ!

My brother in law stormed into the bathroom and demanded sex

I am a girl aged 18 years and I had just been accepted at University to study medicine.

The problem is my brother in law who is an engineer. When I was processing my papers for university I was staying with my sister. Now her husband has been on me for some time now, he wants to have sex with me, he has on several occasions stormed into my room while I am either sleeping or dressing demanding for sex, one day he stormed the bath room while I was naked and demanded for sex, he usually goes for work around 06: 00 a.m, meaning I would be up by 05:00 am to prepare food and water for him, during this period he would be all over me wanting to touch me but I would either run away from him or threaten to scream.
One day around 02: 00 pm my sister was away at the market, we were the two of us at home plus his son. He chased the son aged 8 years to go outside and he started watching a porno movie, when I was passing through the sitting room to go outside he held my hand and forced me to sit next to him to watch the porno movie but again I threatened to scream and I left.
He sent me an SMS saying “Your sister (his wife) since she is pregnant, she produces a lot of water from her vagina and I don’t enjoy sex so I want to be having sex with you and you will always be having money in your pockets,” I have these SMS with me and after I realized that it was becoming too much, I had to run away from town.
I told my mother who is a nurse and my brothers and dad everything what her son in law has been doing, she was and she is still shocked but she advised that we must not inform my sister who was then pregnant saying her BP can easily go up and anything can happen. True to my mother’s words, my sisters BP went up during delivering, she almost died. She had to undergo a lot of medical requirements when giving birth. People my brother in law would around 03: 00 am call me or send me SMS to go to the other room or sitting room so that we can have sex, it was just too much.
Recently, I visited them, they are keeping my cousin who is 17 years, I asked her if my brother in law has been doing the same to her, she said he has been doing the same but that she warned him that the entire family knows his behavior and that we no longer respect him that is how he stopped.
I overheard him on the phone talking to a woman saying “you must come, I will book you a room and I love you very much”. Here are my questions, did we make a mistake by not telling my sister about her husband’s behaviour and could it be that he is seeing another woman?
My mother has maintained that we must not tell my sister about this issue. I still have the SMSes he has been sending to me and I am not comfortable with my brother in law any more.
Sue