Good luck! You can also bring up how responsible you are as you take such good care of your bettas already. =) You could even butter her up more and say you learned how to be such a good fishparent from her example as mom. =D
...Or you could sneak it...do you think she'd know if you suddenly had one more tank...? =P
Well, there still is that 1% chance? yeah and really think, how many people go to the pet store and buy bettas (exept us ;). this one time I went to petsmart (Waaaaay better then petco and closer to me) and looked at the bettas. a week later I went back to petsmart and the same bettas were there.