I have spoken to my boyfriend about it (always gentle-like; don't want to put him on the defensive). At first he completely denied it. But as of the last year or so, he's admitted that she's not as engaging with me, but he doesn't know why. I can tell it hurts him a bit, so I always drop the subject.
I guess her behavior towards me isn't really hurting me. I certainly don't feel accepted by her, and that stings a bit. It also makes our visits very awkward. It's startling to see the difference between her and the rest of her family. They treat me like one of their own, and make quite the effort to remember things about me. That's one reason why I prefer to go visit when there's a family reunion/gathering of some kind, lol!
I guess I've never met a mother that didn't try to express some interest in their children's friends (or in this case, girlfriend), so it's just weird to me that she doesn't. Makes everything a bit off-kilter.
I do appreciate the advice and insight!