If she's fine about it now and isn't being teased because she hasn't developed secondary sex characteristics, you can let it wait until the summer. If she thinks periods are gross and her sister is talking about carrying a baby, be sure that both remain okay with this and it doesn't become a stressor rather than a humorous exchange.
I'm surprised, though, that no one referred you to an endocrinologist to do some preliminary testing of hormone levels. That would be pretty easy to do, and if you're worried about it, I don't know why you would wait to at least do a first pass at some basic tests.
I'd have a chat with the pediatrician to ask about a referral to a pediatric endocrinologist. If you have a children's hospital near you, that's a great way to get a good team together (pediatrician, endocrinologist, urologist). If they aren't all in one facility, make sure they coordinate well by phone/computer so everyone is on the same page. That doesn't mean you have a super-stressful battery of endless tests and in-patient treatment! It means that it might be very helpful to have some baseline levels to compare with new readings in, say, 6 months. You'd see if there were any changes showing internal changes. That could be very reassuring to all.