[yorkiemini]

So sorry this happened to you and Callie.
First we are all responsible for our actions/words - no excuses there.
It is not wrong for you to set boundaries regarding how your dog is treated and to protect her, especially on your turf.
Not being around when your mother comes over may be the best for safety.
Perhaps you could set up some play dates with Sammy & Callie that do not involved your mother disciplining your dog and just let them play. She might agree to that if she trusts you to be kind to Sammy.
If her moods are not stable it may not be safe for Callie to be around her at those times.

Hope things get better for you. Sounds like you really care about Callie and are taking good care of her!