ANSWER:
No, you’re not sinful as you’re not obliged to maintain ties with them because they’re not your kinship. However, you must give them Salam whenever you meet them somewhere. You should also grow up as normal people and mentally healthy people are not affected like this! She is your children’s grandmother and the mother of your husband. Eventually, your attitude will affect your marriage and the love of your husband to you. One must learn how to ignore people’s negativity by smiling in their faces, not defending yourself at all when attacked and not to ever argue with them or reply to their negativity! Once you do this, you would kill them softly and they will be furious for not being able to extract a bad reaction from you or to make you sad.