Mental harassment of my mother by my elder sister

I have an elder sister who does job in a psu in west bengal. I completed my btech and am doing mtech in a govt university but i am preparing for govt jobs. In 2014 my elder sister completed her btech in iit and cam home jobless. While she wrote gate exam 2 months before we were awaiting psu results to get shortlisted but she was still preparing for other jobs> during the time she behaved very badly with my mother taking her for granted like not letting me to the room even for basic purposes and being very lazy and i should not say this but behaving very badly with my mother who was doing everything she can both financially and emotionally.Then after getting the job every now and then she is making my mother a slave using her like a maid for cleaning the quarter that government has provided and there too she always compares the attitude my mother has shown towards her and towards me when i did not get a job through gate. I cant see all this and when i come in the middle she tells me that even i had cared so much for them then i should have got job till now. Every time she mentally harasses my mother and doesnt take no from her today when her training is getting completed she is asking my mother to come to west bengal and do maid work for cleaning the apartment . I have exam this sunday and my mother doesnt gate leave so easily and when she said no she is saying that priority is always me for my mother that is why she is saying no and mentally harassing .From 2014 she recalls the incident and bad mouths both me and my mother especially I am 22 years old and my mother is becoming old she always consoles her and still my sister sees her like a villain.My relatives etc are very opportunists and we cant disclose this to them. I am feeling so insecure about my mother losing her mental balance by all this she is even getting memory problem these. And she obviously will not encourage me taking any legal action against her. Even my mother is saying that she cant take all this at this age she is still harassing i want to put a full stop by no means. Please advise me what shall i do??