Thank you both. I really appreciate the peace of mind knowing I was on the right track. She is ridiculously intelligent and perceptive (which of course has us all frightened for her teenage years) so I wouldn't be surprised if she's already put it together and just isn't sure how to equate it to what she already knows about families.
She also asked that day if Uncle Karma and I were married which led to the garage comment.
I have no issue letting my brother and sister in law lead the talk, but I have a good feeling Karma and I will be the ones explaining it if there are questions.
This has also pushed forward our coming out to my parents. It's been something I planned on doing anyway, but now it's a must. Whatever my niece knows, YaYa will hear about. I'm thinking of taking my mom out to dinner and discussing it with her when we are in town next.
So I'm guessing the best approach is that Uncle Karma has a wife and a girlfriend and while that's not how every family lives, it's good for us (or something to that effect) and seeing if she has questions.