Unfortunately, you are unlikely to see the money again (they'll probably always have some emergency that will absorb any extra cash), and you certainly won't see it before Christmas.
I think you now have to think about whether the relationship is important to you, and if so, how to salvage it.
It is perhaps a poor substitute, but I'd think about what your SIL/her dh could do to repay you in a different way - i.e. could she do your ironing for a (long) period of time, clean your house every week, do the gardening, childmind and/or babysit your dc, do the school run, etc.
If she is unlikely to ever come up with the cash (and she is unlikely, unless there is an inheritance or she wins the lottery!), she can give you her time/do work for you.....sort of like an indentured servant! 
I think it is the only way you won't feel furious every time you see her, and furious every time she/her dh buy (or do) something that is in any way non-essential.