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.
I would talk to a lawyer and see what options you have. An oral contact to repay the money is still enforceable. You can even have the lawyer write a letter to your brother demanding immediate repayment. That will show you mean business about getting the loan repaid. If he still refuses to pay you can get a judgement against him in court. Once you have that you can go after his assets, garnish his wages, etc.
I agree with those who said that, if you want to maintain a relationship with your brother, at this point you have to just live with the automatic payments he's making. And in the future, if you want and can afford to help a friend or relative, give them money. Don't lend it. I learned the hard way, too, though on a smaller scale than you.
I've recently been on the other end of this, where my brother and his wife loaned me a large amount of money a few months ago. I had an agreement drawn up even though he was really chill about everything and I've already repaid it. Personally, I don't think you should ever loan money, you should only gift it. Family member or not. If someone is in the position of needing cash, you don't really know when they will be in the position to repay it, if at all. In this specific situation you have to choose between the money and your relationship. If you want the money, you can consult a lawyer but it sounds like you didn't even have an agreement so you probably won't get what you're after. If you want the relationship, make nice with him and drop seeking interest. That seems like a dick move anyway if you never talked about that before. This whole way of thinking is a bit weird to me honestly, but I think I just have a very different relationship with my siblings.