serenbat- I know how the internet works, thanks.
OP- I'm not upset at all- I'm still getting used to the old TOU not being in place- in the days of yore, we were to *never* post links to other posts.
I think you're in a tough situation, but there are going to be lots and lots of tough situations. In the OP, there was no indication that you were being asked to cover for FIL's GF *to* her husband and kids. I agree that changes things somewhat- it's a crappy position to be put in. As others have said, you are kind of stuck.
That said, your feelings and relationship with your FIL's GF is separate from your kids' relationship with her. Refer to her as "FIL's friend," and be nice to her. That's all you can do, short of cutting her out of your kids' lives fully.