If my late father owned a house 50/50 with my older sister and I'm assuming his half went to my mother who died last year, do all the children now own that half?

You will have to consult a Pennsylvania lawyer in your area to answer this question.  The question depends on how the deed was written to your father and sister and how your father's estate was (or should have been) distributed.  Your father's 1/2 of the property did not necessarily pass to your mother upon his death.  It may have passed to your sister.  A lawyer needs to look at the property records and ask questions about the estates to figure it out.