SUBREDDIT: r/relationships
TITLE: My [22 F] girlfriend tells me [24 M] all her family's history, 3 months, what's right about that?
POST: I've listened to all my girlfriends troubles. But lately the more she talks about how ashamed she is of her sister sleeping around or the craziness of her family, the conversations we have start dragging down into the mud. And we talk for nearly 2 hours a day on phone. 

I barely tell her any of my family history problems for fear that we'll just be two people who talk shit about others and wallow in despair. Sister's divorce, sisters being shitty roommates, mom's crazy. She hates certain people and tells me I should be aware of them. She thinks it's lying if I'm omitting information, but I think what information is relevant (cheating) is what makes it lying. 

Maybe she's too honest. She told me that in an alternate reality she would be dating my brother and I would be dating her sister. That makes me pretty mad, but I laugh it off because then the mood would be very tense. She's very kind and funny, but it makes me wonder why she doesn't just use someone else as her vault of secrets. I'm not expecting anything out of her telling me, but I did ask her why she tells me these things, and that made her question if I regret hearing them, and it could affect how we will talk in the future.
TL;DR: 
Girlfriend tells me possibly too much information where it's even negatively affecting our interactions. How common does this happen? What to do about it without making an issue of issues an issue?