My dad likes fish, so he doesn't mind if I get a few. I have two tanks, and that's pretty much my limit. My mom on the other hand doesn't want me to get any fish because they die faster than a dog or cat. The gourami my dad has has outlived one of my dogs so far... The dog died of natural causes at 5 (20 days away from 6), while the gourami is ten? We got him/ her awhile ago.
Just be thankful your mom let you have two bettas. You have to remember, she didn't have to let you have those bettas. The best way to show that your ready for another betta is to not ask anymore, and take great care of the bettas you have now. Once she sees that you take good care of the ones you have now, she might rethink letting you have another betta. If you ask again, and again it will get on her nerves, and she will just get mad, and not want you to have any more bettas.