I would call up MIL and tell her that you had planned to make them a quilt for their new home for their anniversary. What color is their new
bedroom, etc. With any luck "your" quilt will go. If it doesn't go, I would say something like "I hope that I can finish it in time" and "I hope you will understand if I can't get it done in time for your anniversary"
She may tell you that she already has one for the bedroom, but that she
would love a throw for the den.
Either way, you'll know what colors to work with.
Good Luck - been there, done that!!