I'd like to understand exactly where you're coming from with this. You say you want to make casts because "they'll fit the bra". Frankly this sounds like a bit of rationalizing to me.
If you bought your own bras you could choose the cup size and have any bosom that you wanted - within reason. And it would be you. But instead you want a particular breast that happens to belong to your partner. Do you want to be her?
I'm not saying there is anything wrong with that, but it would be best to be honest, both with yourself and with her, if this project is going to go anywhere.
