I am glad you think it is funny that her brother is the father of her babies.
When you have seen the devestating deformities that can be caused by inbreeding you wouldn't find it so funny.
If you continue to let your cat poke her nose in the hutch you will be lucky if the mum doesn't kill the rest of her babies.
I guess if it was the brother that got her pregnant that the poor thing is only young too.
I wish your rabbit luck having an owner like you she is going to need it