Does a new power of attorney make the initial one void?

Yep. If the lawyer felt she was not capable of making that decision due to incompetence, he is bound by law to have her complete a competency test.

POA's change hands like crazy. Esp as time, relationships, emotional issues crop up.

Sorry sis is being this way. Did y'all have a falling out? In honesty, people generally DO NOT understand the role of POA.

PLus which, it is a pain and a half.

If you have issues with sis doing this (and admitedly it was kind of sneaky) TALK to her.

I'd discuss it with an attorney in your jurisdiction. Normally, the new one will say that old POAs are revoked and the new one stands, however, the new one would only be valid if the person was competent to make it. So, it would depend if she was competent. The attorney who drafted should have inquired. I'd check with a different attorney to find out your options, if you have doubts or problems with it.

Yes, unless there was coercion involved. Then that would be a legal battle.