What do you think about me telling my mom we are no longer on speaking terms after she gave my sister an ultimatum.
What was the ultimatum from your sister's side that she got from your parents?
The ultimatum was for her to call me.
While it is understandable that you can be upset with a parent over their behavior, it's not acceptable for you to give a parent the silent treatment as that is very rude and inappropriate behavior. You are expected to work out any issues with a parent through discussion and not through means of petty revenge like stopping talking with them. You should try to talk with your mom about her behavior rather than let the issue fester and strain your relationship any further.
I will call my mother tonight and tell her I no longer want to talk to her.
At least you're not ghosting her entirely -- that would be wrong. It's important to hear her side of the story. I bet you'll find her heart is in the right place, if maybe a little misguided.