look up loca parentis Penssylvania and you should get the answers
My name is Stephen R. Cohen and have practiced since 1974. I practice in Los Angeles and Orange County, CA. These answers do not create an attorney client relationship. My answers may offend I believe in telling the truth, I use common sense as well as the law. Other state's laws may differ.. There are a lot of really good attorneys on this site, I will do limited appearances which are preparation of court documents it is , less expensive. However generally I believe an attorney is better than none.
First, it is "In Loco Parentis", and there is no actual statute. You will need to look at case law, since the idea of "in loco parentis" is a legal theory, upheld by case law. Grandparent standing is governed by 23 Pa.C.S.A. §5324 and 5325. My suggestion? Get a lawyer.
Second, the step father likely would not need "in loco parentis" standing since he is a grandparent. Blood related or not, he is in the same position as would be the actual paternal grandfather. If, however, the father's parents were divorced, that may change my answer.
Here's the link to the statutory law. You'll have to search the internet for the case law or, as Mr. Koense suggested, retain an experienced family law attorney to do it for you.
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