Well, if it was already fully resetted, (you had to re-do the date, birthday, time, etc.) then I'm afraid it's already too late. You'll just have to wait until it grows up again. You can also do what madi! and Iamtamagotchi said: when the egg appears, press the B button and select DOWNLOAD. Your Tamagotchi should come back to it's last big event; getting a job, going to school, mating, etc.
Or...
you can debug it, choose the adult that your sister had, and that's it. But I highly suggest that you don't debug (I was just giving you an option). It can break your Tama if you don't do it right.
I'm afraid those are the only options available. Correct me if I'm wrong.