viva, Hugs and warm wishes to you to move forward through such a challenging time.
I wonder, can you have your sister frame a photo of you (closer up is better) and each time you call, make sure your sister or someone has the photo available for her to see? Just thinking how important those visual cues are as someone said.
Also, can you look into B-12 Methylcobalamin supplementation for her? It's supposed to be quite beneficial as is D3.
And I totally agree with trying to keep the conversation "happy" for your mom, not trying to check to see if she remembers something (as hard as that must be). I've read that Dementia is like aging backwards and the adult children can often cope better if they think of it like this.