Sorry Jeffy-poo, this one might sting a little.
Imagine a world where S20 is brainwashed and decides to 'take sides'. I know it seems impossible, but the best thing you can do for S20 is love him enough to IGNORE W's outrageous behavior. If indeed, she is circling the wagons, and you have raised your kids correctly, S20 will eventually see through the charade. And sometimes, even when you raise them right, they don't make good decisions.
IF S20 agrees to help W and gets used, he will learn to ask more questions before jumping in.
IF S20 helps W and later regrets it, you will be there to pick up the pieces. It's who you are.

Listen closely...Quit trying to figure out what SHE is going to do, because she's going to do it anyway, with or without S20's help. Instead, take that energy and plan your next adventure. You must GAL, and you must do it soon. Your kids are watching.
Peace.
p.s. The worst part about Thanksgiving was that S18 spent it (voluntarily) with STBX. He's a very good cook, and did turkey w/ all the trimmings. S18 used to be my best friend, I miss him and what we shared. He called yesterday to tell me he will be there for Christmas Break as well. Ugh.