I'd tell your MIL privately how Don's presence at family functions and mannerisms bother you. One-on-one...and I'd stick to how it makes YOU feel. Leave mention of anyone else out (including your husband, FIL, and any other siblings) ~~ after all, you are really only qualified to speak for yourself.

I wouldn't ask for immediate remedy, I'd just let her know how uncomfortable you feel, and that if it keeps up you may not attend many family functions. Let her make her own decisions.

Above all else, I'd avoid being accusing. Talk about how YOU feel, not about how wrong she is...or anyone else.

My 2C

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Life is too short not to love and be loved....preferably multiple times in one night.

I think men talk to women so they can sleep with them and women sleep with men so they can talk to them. ~ Jay McInerney