Ehh, this is really normal for that age. I think it's actually really good that she felt comfortable enough to ask! I remember when my dsd was kind of asking questions right around that age, and then suddenly had a light bulb moment when she looked at me, looked at her siblings (mine and her dad's kids) and then blurted out, "Wait, does that mean you and dad had sex?!" Awkward! But I think my response was something like, Well, yup, that's how babies are made! and it wasn't a huge deal.
If she knows you and her dad are in a romantic relationship (which I assumes he does at this point!) I think it's probably a good idea to have a quick chat with her about how sex is a normal and healthy part of a romantic relationship between 2 consenting adults, etc. Nothing wrong with that, IMO, as long as she is asking and you aren't just randomly letting her know about yours/her dad's sex life.
Single mama to dd and ds , loving my dsd always .