Bad sister in law

My sister in law is bad and jealous and give me and my husband bad treatment.She called me and said I'm not going to leave you means if you visit here I will show you( don't know what she wanna do with me ?) My mother in law loves her daughter more than her son. She has always given priority to her daughter. Her daughters are 52 yrs and 51 yrs old. and son is 6 yrs younger to them. They both had intercast marriages and after marriage they (Manisha and Parita) used to stay at her mother's place as my mother in law used to cook for them. They are lazy. My mother in law took care of their kids. After my marriage we used to stay outside of Mumbai and finally we settle abroad. Once my FIL had surgery so they took care but we paid money for FIL's surgery. Now my sister in law wants their parents flat on their name as they took care of them and they gave ultimatum to their brother that you are not related to his parents and them. They had big fight and said that don't ever visit and stay at his parents house. They completely ignores his brother and talk bad about him to all relatives.
Now my question is how to behave with them I mean should we give money to parents now onwards thinking that my mother in law is going to give everything to them not his son. Can we claim our share as we gave monthly expense and money for surgery for MIL and FIL to them? My mother in law never liked his son since birth as she has physcologyical problem like she had to take care of her parents along with her sisters and her brothers did not took care of her parents.He is unwanted child since beginning. She used to say after my marriage that you guys are not going to take care of me, I know only my daughters are going to take care. My sister in laws always fight with his brother and says bad words to him and our kids.