I'm so sorry you're dealing with this. I really do feel for you. I am both the wife of a man with ADD and the daughter of a woman who sounds much like yours. It sounds like you really want to connect on a deeper level with your family and it's not happening for you. I know exactly how lonely that feels.

I can tell you that there's much more to ADD than constant talking. I can also tell you that I chased a relationship with my mom for almost 3 decades until I realized that it just wasn't going to happen. From your brief description it sounds more like your mom is self absorbed and your sister was raised by her so it makes sense that some of those traits would be passed on.

The first thing you need to do is remove all expectation. That sucks. And it's hard. But you have to if you want to save your sanity. There is a very decent chance that neither of them will ever wake up to what is going on. You are only making yourself suffer by chasing a connection that they aren't willing to work on. I had to realize that I do not have an attachment to my mother like most people do and I have to get those connections elsewhere. I can't talk to her about anything. She hears what she wants to hear and misses the point half the time. It's more work to get a point across than it's worth. So I let her go on and on about whatever the dog did that day and I don't try to get deep into what's going on with me - it's pointless. She wasn't excited about my wedding. She still, 22 years later, talks smack about my dad from whom she is divorced. There's just no getting through.

The second thing I want you to do is read The Four Agreements. It's about personalization and realizing that everything other people do is about them. It's really important to realize (maybe you already have) that anything going on with them really is only going on with them. It sounds like you may already know that but it's a good book to read anyway.

You know them better than anyone on here does. You have to decide if your time and energy is worth trying to get through. And if you think they'll respond positively. You could be surprised. Maybe you call your sister up and say "I really just need to talk and have you quietly listen, and REALLY listen, not just waiting for your turn to talk" and she may think "oh my god I can't believe how rude I've been!" Only you can gauge that.

Anyway, I hope you figure something out. I know how lonely it is wanting so badly to connect with a specific person and not being able to. Honestly, when I came to the realization that I would never have that with my mom I spent weeks intermittently crying. I was grieving. I grieved the relationship I never had growing up and the one I tried so hard to have an adult. Good luck and know we're here for you