Husband's married sister permanently staying at her patrimonial home and torturing me

I am a 27 year old woman who got married last year in the month of March.I had a blissful marital life with my husband who is a scale 2 officer in a psu bank.My husband and I were staying in Mumbai as he was posted there.I also had very cordial relation with my in-laws so far who stay in W.Bengal.My husband recently got transferred to our hometown,Siliguri and during that very peak moment,my father in law suffered a cardiac arrest and expired in June,2017.My life has drastically taken a cruel turn after this incident.My husband's sister started torturing me from the next day onwards.She was my school friend.She is married since 4 years but does not stay with her husband.She stays with my in laws (her parents.)I was not aware of this fact before my marriage.She is issueless and the relation with her husband is like that of a dysfunctional family.Her husband is completely cut off from his own parents as serves as a servant for my mother in law and her daughter.He does odd jobs to satisfy his wife and mother in law for the sake of inheriting huge property.My husband's sister is jealous of me because I am academically bright and I am a trained Classical dancer,because their relatives appreciate me.She was abusing me verbally every day from morning to night while I was performing the last rituals for my father in law for 11 days.She also started brainwashing my mother in law who started harassing me in every move I was taking.My in laws are quite affluent and though they have a maid,I was made to wash utensils thrice a day,prepare all the food alone though I was suffering from low bp and it was known to them.I fell down due to weakness,vomited went without food and every time my husband would come to me,he was stopped by his mother and sister.Actually my sister in law could not tolerate the fact that his brother got transferred to his home and so naturally I would be staying with him in his home permanently.She used to shout and tell my husband to kick me out of the home.My mil also started supported her in every action.She started not allowing me to use my washroom,came to my room and switched off the fan,saw me vomiting and going without food,tried to snatch my phone, forced to do all chores.My parents visited and told me that they would take me to a doctor.My sister in law locked the gates and started shouting,calling abusive names.My mil came and pushed my father,pushed me against an iron grill gate,pulled my hair,scratched me,verbally abused my mother.They kept us locked for an hour until my husband opened the gates.I have medical proof of assault.I am staying with my parents since 3 months.Initially,my husband was texting me.Now,he is not.He visited my home this month and told that his uncles will discuss the issue.My husband is totally brainwashed so he is not inquiring about me now but basically he is not bad.I want to stay with my husband and mil.How to teach her(sil) a lesson and preserve my marriage?