I have been in the equestrian business for 20 years, and unless you are running a multi-million dollar show facility, running a barn just isn't profitable. The horse industry is dying, and dying fast. Those who keep barns alive do it because they love it, not to become rich.
It sounds like this farm is in some serious trouble, and I wouldn't count on the economy turning around and helping it.
Here are some questions I would ask to see if there is even a chance if this farm can succeed. If they cannot answer these questions sufficiently, you need to convince your fiancee to walk away, she will only drag you both down with the failing farm.
How many lessons a week do they teach? How many horses are leased? A profitable farm does at least 5-10 lessons a day, 6 days a week. Ideally, at least half of those would be group lessons. A good farm aims to have at least 3-5 horses leased at one time-that can pay the feed bill easily. Even in bad economies, people love leasing, and you can actually make more than you did before the economy tanked. People who no longer can afford their own horse miss the experience, so they settle for leasing.
How many GOOD lesson horses do they have? A profitable farm has a school of at least 10 solid horses who are reliable, healthy, and athletic. Less than that, and the horses are overworked and shabby looking, which means no one wants to go to that farm.
How much boarding space do they have? If they have less than 10 stalls for board, they will fail. Board bills pay the mortgage. Small family barns crumble fast.
A profitable farm makes its bread and butter on 1) Boarding then 2) Leasing, then 3) Lessons. If lessons are their priority, they will never make money.
From what I've read here, this family business just isn't sustainable. It sounds like they're in way over their heads. I would strongly encourage the fiancee to move on to a new job, or for her family to sell it.
If that's not an option (which is crazy that she would think it's a good idea to only pull in $18-20K) then you'll need to pull the extra weight by taking a second job and cutting expenses.