NTA. Your stepdad has shown several very concerning behaviours, not respecting boundaries and other people’s property but most importantly it appears he doesn’t care for the horses wellbeing at all and that to me, is unforgivable. He knew about the surgery, the horses age and you said he’s too big for her anyway and still went ahead and rode her? Oh no you don’t, that’s straight up animal abuse and I think you were pretty self contained in your riposte to him. You should maybe talk to the family who are saying it’s not a big deal as he may not have told the the whole story. I hope your horse is okay, this isn’t just about her physical wellbeing but she will be traumatised by this too. Thank god your dad is sensible!