Good news! We received an e-mail from SIL today. The check they mailed us today is for almost 25% of the total (which is slightly less than the original amount promised but enough to make us happy). They will pay the balance off in equal monthly installments that will have us paid off by the wedding (November).
This was all offered by her without prompting from us, and is much more like the SIL we thought we loaned the money to! Now, all they have to do is live up to it. :) How well they do that will determine their future credit standing at Bank of The Weird People Next Door.
Thanks for all your tea and sympathy. (Over the weekend, DH and I were discussing this situation, and I told him I had spared him many of my thoughts because I knew it would just keep him boiling. He said "Yeah, but I bet your friends have heard all about it, haven't they???")
Nurse Ratched (remove "cuckoo" from address to reply) We'll all get back to normal if we put our nation first, But the trouble with "normal" is, it always gets worse. ~Bruce Cockburn