Yes, everyone wants their family to be happy and healthy. But in your case you have a cancer in your family unit called the Watchtower Society. It has destroyed more families than you can imagine. If it doesn't destroy completely, it almost always ruins portions of a family.
I still maintain a somewhat 'cool' realtionship with my mother. But my sister cut me off some twenty years ago. I lost all contact with my two nephews and niece. Now, they're all grown up and I'll never get the chance to be that uncle to them that I could have been. A few years ago we had a little girl and suddenly my sister wanted to be sweet and loving, so SHE could have a relationship with MY kid. Well, screw that. I let her know I wasn't interested in any kind of a relationship with her and any idea she had about getting to know my kid was out of the question. I just won't allow any JW influence in my kid's life so she'd be out of the picture anyway. I only go to see my mom every couple of years (she's out of state) and I NEVER leave my kid unattended with my mom. I know she'd take any chance she could to talk JW to her (my kid). Not gonna happen.
Don't deny your feelings. You're angry and you're pissed about the way your sis' is treating your mom. Why not let sis' know how you feel by shunning HER? Afterall, she seems to think this is a practice that is acceptable, right? Seriously though, you have to decide what's the most important thing that you want. Do you want a relationship with you sister? Sure you do. Do you want it at all costs? Are you willing to have a relationship with your sister while she refuses to have a relationship with your mom? I'll bet you don't want that. Let HER feel what it is like to be shunned for a while. I wouldn't call or wright her a letter or anything. Nada.
Let me tell you a hard cold fact. There are going to be people in your life that you are going to realize that even though you may love them, they have an overall negative influence in your life. And there is nothing wrong with cutting them out of you life. It can be painful and difficult, but at least you know where you stand. There are those here that may disagree with this advice (maybe not very loving or something like that). Some want other's to make peace at all costs to 'save' or repair the family. It seems like the right thing to do. And maybe making peace or trying to keep the peace in your case is the best thing to do? Maybe not. Every situation is different, so you can't make a formula that every one can use. I only know what works for me.
Good luck,
Steve