Legal Question in Wills and Trusts in Pennsylvania

Life insurance and joint bank accounts or payable on death (POD) designations on bank accounts are non-probate assets. That means that they will pass to whoever is the designated beneficiary of such assets which would be you in this case. The designations cannot be challenged as between your sister and the bank or insurance company. However, nothing would stop your sister from suing you IF (and this is a big if) she believed and was able to prove that you somehow exercised improper influence or fraud here. That would not be a likely scenarios though.

Power of attorney ended at death and if your mother had no power of attorney while she was alive then you obviously could not have used the power of attorney to make yourself the beneficiary of the life insurance or bank account. Which makes it even less likely that your sister would sue.

You indicate that your mother died without a will. What other assets did your mother have? A house? a car? Personal property? Since your mother had no will, if your mother had no surviving spouse, then all of your mother's other probate or non-beneficiary designated assets would pass to her estate and would be split between you, your sister and any other siblings.