Stace, don't let the MIL get you down.
Think of it this way...she's older than you, meaning that she'll most likely die before you do. You can always celebrate silently! LOL!
That is unless she doesn't actually die when she's supposed to and ends up moving in with you and yours so her diapers can get changed on a regular basis.
Things could always be worse - you should be glad she goes away when she goes home!