Still living at home, I also am finding myself in that situation. She knows that I want to start a breeding program, but she is quite afraid of snakes. I actually was quite pleased she let me keep two in her house, so I can't really push the issue (free rent) But like myself, just talk to her and have a good reason on why you think you need to purchase more snakes. I've told her to look at it as an investment and extra income for myself.
I stand now with a 1.1 pair of normal BPs but as soon as I get the A okay things will be moving along. I've slowly gotten her to come face to face with her snake fear, and now she can actually be in the room with them in the cages. She actually suggested making one of our spare unfinished rooms in to a reptile room, so I'm thinking she's opening up more to the idea of more animals.
Just don't push the idea on her too much, it makes parents irritated.