How much more clear can I make it?

Here's a little background: my mom and stepdad have been married for 20yrs; since I was 10 yrs old. Since my real dad died 6 yrs ago, my stepdad has dutifully and proudly taken his spot. Well, my parents are divorcing. I'm 30 yrs old and have a marriage and family of my own to maintain. So I made it very clear to them both, in a polite manner, multiple times, that I don't want to hear either bashing the other. That's what counselor and attorneys are for, not kids. My stepdad has respectfully kept his end of the deal. My brothers and sisters from his first marriage also respectfuly stay out of it, or at least don't mention it around me and my family. My mom on the other hand is impossible. If I text her and ask how my 11yr old sister is, she will bring him up. If we're on the phone talking about my daughters upcoming birthday, she rudely brings him up. Last week my sitter was sick, so my mother offered to fill in, great. I told her that all I asked was that she NOT fight with my stepdad all day on the phone while she's at my house with my daughter. (that's what they do while my sister is in school- fight on the phone or text.) she got mad and told me I had no right to tell her what she can and can't do around her granddaughter, then refused to babysit. I again told her I don't appreciate hearing her bash my stepdad and that it's disrespectful to do so in front of my family. Yet she continues to call and text my cell to tell me something rotten about him. I hate to completely cut ties bc my daughter still needs to know she has a grammie. And I don't want to abandon my little sister. Am I wrong to just completely chg my cell phone number, not give her the new one, and tell her to call my home phone if she wants to talk about one of the kids? How much more blunt can I be? She's talking to a counselor, and her pastor. I can't handle listening to her issues and maintain my own healthy marriage!