Hello.
To be honest...the ONLY thing you can do is get bruvver, muvver, gf and yourself in a room and talk.....as there is a one on one situation going on.
Take pics of the rugs, field etc...and try to sort it out.
Best of luck.
Bryndu
Your brother sounds as if he needs taking in hand by your mother tbh. He needs telling firmly that his actions and lack of responsibilty just aren't on, and he either pays and does the work, plus what he already owes you, or the horse goes.
If he doesn't take resbonsibility then I'd sell, and two fingers to anyone who makes you out to be the bad guy.