Maybe a little harsh but the whole situation looks to be messed up.
From your mother's initial involvement to backing out and not wanting to be involved now, to your brother borrowing so much money without paying it back, to you wanting to charge your brother interest.
I certainly don't agree with what your brother did, but reading your post, and again this may be a bit harsh, you didn't come off that well. Honestly, when I got to the point where you started talking about providing a "service" I figured you'd have them do your laundry or polish your shoes... indentured servitude like.
Now, if your brother's partner is a hairdresser then they should have come up with the idea of doing your hair gratis on their own, that would at least show their understanding of the need to repay you.
It sounds to me like by now you are too hung up on the money to let it go so I don't know how much good most people's advice to just forget it is going to do. If I were in your position I would probably feel the same way you do, but I don't think I would ever lend a family member that much money, I would either say no, or give it to them outright.
I think you are petty much boned unless you can sit your brother down for a straight talk and figure out what he plans to do, if anything, about the money he owes you.
You need to get some kind of closure, and just writing the money off isn't going to provide that because it will fester under the surface and forever poison your relationship.