Hypothetical situation: what would you do?

I would give her time to think. She is the rightful mother, and if you don't even believe in having children of your own, then you shouldn't be quite ready to take care of the baby. Besides, it would be very, very weird for the baby to grow up with YOU who is your neice's aunt and just... It would be very complicated. I would let her decide, but if she decided not to want it, then you would take care of it.