Though, we know and teach kids that we should not lie, but in situations like such, we have to tell them something that's not true as there is an appropriate age to give children certain information.
I guess, you can tell her that when mamma feels lonely and wants a little princess or a prince in her life then she wishes for a magic so that the baby princess or a prince comes in her tummy. And now, mamma feels that if there be another baby you will have a company, so mama wished for another baby like you. I mean, something like this you can tell your daughter. I hope this one settles her curiosity for now. :)