I thought this one was interesting because I can potentially see controversy and different views on the girlfriend's actions.  Though I suspect the universal answer is the LW needs to bail, if this is a dealbreaker.  The letter:
I have been dating “Rowan” for almost six months. I knew Rowan had lost her parents really young, but I just now found out she has a sister. Basically, their dad died in an accident when Rowan was 13 and her sister “Rue” was a baby. Then when Rowan was 20, their mom got cancer and died a year later. Rue was 8 and Rowan allowed her to be put in foster care. Rowan wasn’t in college or anything, she’d been working and supporting herself since 18. She says child services tried to place Rue with her, and she said no. She says Rue was a nice, normal kid and she didn’t hate her or anything. She just wanted to enjoy her twenties instead of raising a kid.
Rowan seems to think this was a totally normal and okay choice to make, and doesn’t get why I’m horrified. Currently Rowan and Rue are in touch but not close. Rue aged out of the foster system a year or so ago. She’s getting by on her own, although from the sound of things she’s possibly some kind of sex worker. Rowan doesn’t seem to want to discuss Rue. I can intuit, though, that Rue blames her for how hard her life was in the system and is now, but Rowan doesn’t accept that blame.
My biggest issue is I’m about 70 percent sure I’m going to want at least one kid in the next 10-ish years. Before I found out about Rue, Rowan had said she’d be down with that. But is there really any chance she’d be an okay mom? Do I need to worry about her abandoning our kid if something happened to me? How can I begin to talk to her about this? And should I even try? Or is someone who would do what she did, and still defend it years later, a terrible person I can maybe enjoy sexytimes with, but shouldn’t plan a long-term relationship?