Wow, what a nasty family member.
Certainly, she owns the copyright to pictures she has taken. And, if she inherited her mother's estate, she would own the copyright to photos her mother took. You can not "obtain" a copyright. You either have it or you don't (okay, you can sell copyright but that's not what is happening here).
Note that technically she can not use any photos, even if she owns the copyright, without model releases for every person. If you want to counter her silliness with more of the same.
The real issue here isn't legal, it's personal. She doesn't want anyone else to have these family photos, at least not without her express permission. And, honestly, I would not even engage her (or anyone else) about the "copyright" business. It's irrelevant. It's about her and her control over this family resource.
Now, rereading your message, it's a bit unclear to me who has what. Is it the case that a woman in your family can't get access to her OWN mother's photo collection because someone else in the family grabbed them and is claiming copyright? If so, then, no, they can not do this legally, except for pictures the person in question actually took.
So you have some issues here, to put it mildly. I see no way out of this without offense because the person with the photos is being offensive. So I wouldn't worry about questioning them, etc.
Perhaps the best method here is to get a friend who is a lawyer (if you are lucky enough to have one) write a letter on behalf of the daughter saying the pictures are part of mom's estate and the estate is in daughter's hands (if true). Chances are that will be enough to scare them into giving them up.
Good luck, you're all going to need it!