How can I find out if my sister really had power of attorney over our mother or not?

If she was a legal PoA your sister would have had the paperwork, as she would have been required to present it in certain cases (like at the bank). I agree with others that since she is passed, it no longer matters. Your mom, if she is cognitively sound, can now create a new document with a new PoA. It's very important that she do this, as well as creating a Living Will (will outlines how much or little "heroic" medical care she receives if she is no longer able to make those decisions herself.)

When your sister passed her POA, if there was one, was no more. So it really doesn't matter if she did or not. If Mom is competent, she can assign you POA.

Even if she did have some authority granted by your mother, since she's no longer living, it's irrelevant.   It may be that your sister didn't understand what being a proxy meant, and that the joint account was what she thought enabled her to act on your mother's behalf. 

But, have you searched your sister's home?