N.D.
Wow! Talk about a difference. You have a newborn and a daughter asking about sex. YIKES! I don't envy that. :) Anyway, now is the time to form a strong "woman" bond with your daughter. Embrace that she has come to you and that she trusts you to ask and for the answers. Kids today do know more than they should and so you shouldn't tread lightly. Be prepared to talk about it ALL but let her lead the questions. Use your son as an example of what happens when sex is right and good. But that sex could also happen under the wrong circumstances and cause a lot of heartache.
I would also take the opportunity to talk about respect, modesty and self-confidence. Ask HER questions about how she feels about her body and ask her what things she respects about her body. Have her do a lot of talking so that you have an idea of where she is going with things and where her head is.
Good for you for taking the time to do this. You will do great!
N.