I think your dad's girlfriend needs to rethink why she is getting a bunny in the first place. Does she want to get a baby bunny because it is cute? They do grow up, and will she then get bored of it?
You're doing the right thing in wanting to adopt yourself, but you really need to convince your dad's girlfriend that adopting is the best thing to do as well. Especially as there are so many pairs of rabbit wanting to be adopted who sit in rescue centres for years sometimes. If she does get a baby and you adopt, a rescue can sometimes help with the bonding, but you'd first have to wait for the baby rabbit to be old enough to spay/neuter which is expensive, then wait for recovery and the hormones to die down. Then you would be able to start looking at getting them a friend from a rescue. It's just not worth the hassle.