I don't really think this is a copyright issue in the end.
Legally, I suspect that copyright belongs to the professional photographer, or their heirs. These heirs may have no idea these pictures exist and there may be no evidence to show who took the pictures. See below for someone's opinion on how this works in the USA, where I gather you are. I don't claim to understand it all, but I am in England.
The real problem here is that possession is nine-tenths of the law. The person in possession of the physical prints does own them (the prints, not the copyright), unless the other members of the family can show the prints were taken from the estate of the passed generation contrary to law, which is highly un-likely. Being in legal possession of the physical prints, they can, I think, refuse to hand them over.
This is, of course, really an unfortunate consequence of poor family relations, not really about the law.
Because of my research in to my own family history I have come in to possession of most of our family photos (with the agreement of other family members), some dating back to about the 1880s, so this point of law came in to my mind at that time. In actual fact, I immediately came to the conclusion that I would let any family member who wanted them have copies of the pictures, even if getting copies made ended up costing me money. It just seemed the right conclusion to come to.
Disclaimer : You may need a lawyer to understand this :http://www.legalgenealogist.com/blog/2012/03/06/copyright-an...