how should I handle sister in law

My MIL favors her other daughter in law. They talk by cell and email all day. They gossip and talk about me and my flaws as a person. How do I know? I just do b/c I will say one story to my SIL and then my MIL will ask a question. Over the years I have been able to put two and two together. I wish I could have this mother daughter relationship that they have but they seem like they rather just have each other. My sister in law had children first and when my husband and I had our first child a few years later we could not believe how much my mother in law favors one grandchild and spoils that child with materialistic things. My MIL did not help me at all when I gave birth to my second child but made comments when she came over that she was able to keep her house clean and she did it all by herself. BUT when my SIL had their second child my MIL stayed with them for weeks to help… doesn’t seem fair does it? My DH even admits that his mother is two faced and has always had her “favorite” child… her “favorite” daughter in law.. my problem is the last few years I had distant myself from my SIL we were at one time pretty good friends but I found out that they gossiped about me and saying very hurtful things. I really can’t stand to talk to my SIL now and gives me anxiety and I really don’t want anything to do with my MIL but at the same time I do want my children not have a grandmother in their lives. How can I keep real and authentic? I feel like I keep sweet when I talk to my SIL and MIL to save problems. This is definitely not worth bringing up to my MIL she is stubborn and will not admit to her doing any wrong and she will go on and on that I am too sensitive. I just want to end ties with my SIL.. how do I do that gradually? I don’t really email her often but the gossip that they do just aggravates me.. how do I handle this and just let it go?