As long as both the biological parents agree, then you should be able to adopt your stepdaughter. And the good news is: having consent from the parent(s) is the fastest way to get a stepparent adoption completed. Some states do require that you be married in order to complete a stepparent adoption, but I do not believe that PA is one of them.
Once the stepparent adoption is complete, you will want to go back to court to sort out custody/visitation/child support issues between yourself and the mother of the child. It will save you both alot of time and money if you can come to an agreement on your own, or with a mediators help. It is wonderful that you love your sd so much and have done so much to stay in her life, and to be her daddy! Good for you!
Stepparent Adoption Blog