I'm half and half on the money part. One one hand, it would be very polite and noble to offer to give them back to the family for free. Maybe you could keep a couple for yourself to sell, so at least you get some money for them. It wouldn't be fair for you to be empty handed.
On the other hand, part of me feels that just because you found something that belonged to a family long ago, doesn't necessarily mean they have a "right" to get it for free. Quite frankly, all of us have items that belonged to a family long ago, they cherished these items and these items also have some good value now. That doesn't mean they have a right to it forever. If somebody brought something to me that my family owned a century ago and they offered it to me for free, I'd feel obligated to give them something for their generousity. I guess it's all up to you, I think there's no wrong way or a right way to go about it!