OP this whole situation is mental. Horses cost what you want them to cost, basically.
I had horses. I wanted all the bells and whistles too but I couldn't afford it. So, [shocker] - I didn't have them! It was 5yrs ago so prices will have risen a bit and I live in a cheap area, but I kept two for £3.5k total per year. Prior to that in an expensive area of the country it cost that much for just one.
Both had health issues, which were managed with herbs, creams, sprays, exercise and good levels of basic horse care. I did absolutely everything myself.
I didn't compete unless I could ride to the venue. Transport was only hired if I was moving to a new yard too far away to ride (so 25+ miles away, under that and they were got fit and ridden there). They didn't have shoes and I deliberately bought horses with hard feet so that I could get away with that, by also limiting roadwork by ensuring I was on yards with off-road hacking and arena or field to ride in.
They had two seconds hand saddles each, one that fitted for winter after the annual diet and one for summer when they'd naturally get quite fat. They never saw a saddler because I've got eyes in my head and could adequately tell when I needed to swap the saddles over. Because the tack was thoroughly cleaned monthly at a minimum, it never broke so it never needed replacing.
Because I took good care of the horses and because I bought tough types, a lot of the types of niggly everyday problems that tend to crop up with horses just didn't. It's surprising how much of that can be avoided by good care, including adequate work after a suitable fitness program and decent first aid in a timely manner. In their lifetimes they saw a vet once and twice respectively for a small cut in an awkward place that may have become infected and twice each for a contagious disease brought to the yard by someone else's horse, which they then caught. Other than that they saw the vet annually for vaccines and every second year they had teeth rasping. Farrier for a trim 4 times a year.
Stabled nights and out days all year round, except for a month or two in summer when they'd be out 24/7 and a month in winter when the fields were shut and they were in 24/7 and exercised daily by me.
No insurance except free public liability cover obtained by joining an organisation and I always knew there'd be no veterinary treatment other than that which could be provided at the yard. Had a disaster occurred they'd have had to be PTS, not operated on and lengthy recuperation. One was PTS due to advanced arthritis in old age and the other in middle age due to lifelong behavioural issues (would have been unethical to sell and not in horse's best interests) when my own health failed and I couldn't keep horses any more.
There's absolutely no need whatsoever to spend £25k/year. It's entirely possible to live a modified version of The Dream and be totally happy. There's no need to work yourself into the ground or bankrupt yourself to be involved with horses, especially when you're not even the horsey one!
My suggestion is the competitions stop, except one outing a year as a treat, then you can forget about the endless ongoing transport costs too. The DC can share a horse, which means owning one big enough for the larger DC and calm enough for the least capable rider. Tough shit if that isn't the horse of their dreams, it's still a horse, which is more than the majority of DC get.
Your wife could have whatever type of horse she can afford the running costs for on her wages from her disposable income after all family related bills (including for DC's horse) are paid by you both. It's literally her hobby, you don't have to fund it. If she's got any sense and kindness, it'll be a horse that at least one of the DC can ride safely occasionally, so the DC can ride together sometimes and not always have to go with mum if they want company. It also helps with exercising the horses if mum is short of time.
Your family need to get a grip. You're not a cash cow and the family isn't rich enough to be able to live The Dream to full and complete effect. It's time they accepted that and stopped being so selfish.