This is like lending money to a friend who says they'll pay it back--it's never going to cross the palm of your hand. I believe the brother-in-law believes the money is "his," since it's money set aside for your neice (his daughter) for college. As far as he's concerned, that money is his or his familys. I know; that's now how you see it, that's not how the fund was set up, and that's not how the fund is supposed to be used. But that's how the dad sees it.
And I do not believe that they "didn't know" that the money for the dance lessons, clothes, shoes, etc., wasn't due the first night of dance. My daughter has taken dance lessons for the past 7-8 years; you always have to pay what's owned the night of the first dance lesson. It's the rule of the game.
Therefore, under no circumstances should you ever "lend" your brother-in-law any money that you expect him to pay back. Because he won't; he/they see it as "their" money; whether it's from their daughter's college fund or not.
If you want the money to go toward your neice's college education, then you must absolutely never give the family any money that you expect to be paid back or "put back into the neice's college fund." Any and all money given to this family from this point forward should be seen as a gift of money--because they've showed you, in no uncertain terms, that they don't plan to reimburse you for "money lended." If you want them to pay you back, then next time, write up a promissary note that will stand up in a court of law, so that when they don't make any payments, you can take them to court.
Or, better yet, just don't give them any money.
I understand perfectly where you're coming from. My husband loaned a friend some money to have some work done on his truck. It's over a year later now, and we still haven't been paid a dime of any of that money. The friend could have put $25 or $50/month aside to pay us back. Or he could stop enabling his grown children with money hand-outs, so he could pay us. Or he could stop enabling a drunk buddy freeloading off him, so he could pay us. The fact is, I don't expect to ever see that money from him.
Lesson learned. My husband will never "lend" this person money again. I'll make sure of that. This friend has taught us that he can't be counted on to repay us back.
The lesson learned is this: never lend money to a friend of a family member if you expect it to be paid back; the majority says, it never will be. If you must loan someone money, then set up a legal repayment plan, have them sign it, you sign it, and a notary public sign it, so that it's legal and binding. That way, when they don't pay you back by the specifics set up in the legal piece of paper, you can take them to Small Claims Court.
Otherwise, you have no alternative but to see this as a gift of money given, get over it, and get on with your life, lesson learned.