Sister calls 95 year old aunt living alone and tells her lies that leave her very confused and upset every day. She also asks her for money.

Do you have POA for Aunt? If yes you can deny contact. You could record the calls, depending on the state 1 party authorization or let sister know that she is being recorded, that might put a stop on causing upset and begging money.

Tell her to put her big girl panties on and go earn her own money instead of mooching off a 95 year old woman. It could be considered financial exploitation of a vulnerable senior.

I am sorry for everone's pain. Was it the aunt who told you that this person is calling her, and that it upsets her? Have you suggested to the aunt that she tell the sister "no" to the money, and she will not likely hear from her much anymore?
Is this the sister of the Aunt, or your own sister? Do you have any relationship with the sister? Might the sister listen you you if you suggest the call is upsetting your aunt?
Other than those suggestions, I think it's important to know that we really don't have control over what other people do and say to still other people. You might need to tell the aunt that if she is hoping you have an answer, really you don't and it's upsetting to hear about.
You are suffering about something you could not conceivably control.
Do you think the aunt might be capable, with an answering machine, of "screening her calls" as we all do with sales calls?

Possibly, Aunt may get upset and sister is ruthlessly terrible.
Can you think of any other way. I see this as elder abuse! Makes caregiving a bad experience.
Thank you for your answer

Can you block her number?