I'm with skoal monster: I wouldn't give 2 shits about what my doctor thought, FIL or not. I'm very close to my FIL. He's as close to me as my real dad, if not closer. I didn't hesitate in talking to him about my quit. Although, he knew that I chewed and had no problems with it, he was very excited and proud that I was quitting and offered up any help that he could. I'm sure your FIL would do the same.
I don't know anything about Xanax but when I told my family doc that I was quitting nicotine, she wrote me a script for Wellbutrin. I was against it because I'm not a fan of meds but she has been my doc for most of my life and she knows my temper so she pretty much demanded that I take it. I can say that without a doubt that it has helped my quit. For me, it doesn't kill the urge, it just makes me not care if I have nicotine or not if that makes sense.
If you are having trouble with anxiety and you don't feel comfortable talking to your FIL, I'd go talk with another doc and let them know what's up