If I was married but separated and my wife died, how do I get her property from my in-laws?

I suggest you call a probate attorney in your area to answer this question with certainty.  I assume you and your wife had not filed for divorce.  I also assume she had no will.  Under those circumstances, you should still be entitled to inherit as a spouse.  Different states have different rules about spousal inheritance and I am not familiar with Pennsylvania rules.  That is why I suggest you contact a local attorney to assist you or answer your questions.