I would call her, or have DH call her, and say,"SIL, thanks for the cheque, but it is way too much!"
She'll likely say, "No,it's not. We really enjoyed staying at the cabin, and we'd budgeted for a hotel we wouldn't have enjoyed nearly as much,"
"No, really, SIL, we can't accept that much. Didn't cost us a dime. But how about this -- anything on top of your share of the dinner, we'll give to Mom and Dad for the water, electric and that?"
She probably loves the cabin and wants to get on the good list for future invitations, and who can blame her? Then tell your parents how great she was, when presenting the $ from all of you. And ask SIL to the cabin again! You know she will pull her weight.