What happens when a someone uses estate property for their own benefit in Philadelphia, Pennsylvania?

You can review the file in City Hall.  Call them first to see if an estate has been opened.  (215) 686-6250.  I doubt that an estate file has been opened yet w/out your signature either as co-administrator or renouncing your right to act as such.  You have many rights in that property and you should protect them.  Call me if you'd like more information.

A petiton needs to be filed with the register of wills in which you ask to be appointed the administrator of the estate. The register of wills will set a hearing date. All heirs will be notified by registered mail return receipt requested and the register of wills willl decide who should administer the estate. The administrator will sell the house, pay the debts and taxes and distribute the remainder to the heirs. The administrator and the attorney representing the administrator are entitled to a fee. Call me 215 496-1044

As an interested party, you should contact the probate court and notify the judge of this situation.  If your sister is wrongfully depleting the estate of assets, or hoarding them for herself, then you may be entitled to a court order rectifying this situation.  Moreover, the probate file is probably a public record.  Therefore, I suggest that you go to the probate court and ask to review the file, and then write a letter to the probate judge if necessary.  Good luck.