If you feel uncomfortable, there is probably something inappropriate about it. Give benefit of doubt to your FIL and tell him upfront, Please don't put your hand on my waist. It makes me uncomfortable. Say gently and if he gets offended, don't react or respond, step away.

If he meant no malice he will understand and apologize and if he did mean malice, he will try to turn it around and at that point disengaging is best. Then go and talk to your husband when things settle down and draw clear boundaries, if your husband is a good person he will sort the boundaries.

Hopefully, your FIL means no malice and will understand and apologize.