My siblings rarely call or check on Mom. She asks me if they are mad at her for something. What should I do or say?

I think you shouldn't "cover for" your siblings. Don't make excuses for them. Don't criticize them to her, either. How about, "I'm not really sure what is going on there, Mom. Let's give Sis a call and see. You could ask her if she is mad at you if you want to."

As JB says, the telephone works both ways. Help your mother place the calls if she needs it.

One thing I do with my mother is tell her to call them. Sometimes our elders wait for their children to come to them. I guess they are afraid that they will be bothering them or something. Encourage her to call them. The telephone is a 2-way thing and no normal person minds their mother calling to check on them.