My 90-year-old mother's out-of-state sister died and I haven't told her. My mom asks if I have heard from her son. What should I do?

I agree with Ed, if she is in stable health and wouldn't over react to the news, it is best that she knows. It would be worse if she found out another way. Just keep an eye on her and be at hand...allow her to grieve in her own time and way.
so sorry for your family's loss.
Lilli

NANCY:

Unless she has a serious heart condition, and whether you're not sure about her response to bad news or not, don't hold back anything back from her.

People like your mom are a lot stronger than they seem, usually accept death as a natural part of living, and are well aware no one lives forever.

Keeping things from her is a disservice to the poor woman, and chances are she won't forgive you for it. So take a chance.

-- ED