I strongly advise you do something sooner rather than later since one day she may just reach your personal threshold & go off on just one little thing she may say or do.

You can either a).Talk to your MB about her + let her know how much all of this bothers you or b).Speak to the sister-in-law directly and let her know (kindly) how her comments make you feel.

I prefer option A since she may not relay what you say to her in the kindest way to your boss.

Good luck!!

What chores, OP?

And I agree you should tell the sister to go ahead and finish whatever job she is criticizing.

Tell her to have at it. If she says you're doing it wrong day here you go, clearly you're better at this than I am so I'll let you do it while I go Do X. Guarantee she'll stop.

Ignore her.

I'm sure she's jealous she can't afford you. Next time she makes a comment, ask her if there is something she'd like to talk about. Call her out. She's not your employer and it sounds like you have a good MB who will back you up.

I get along with my NF very well, but it's MB's sister in law who ticks me off whenever she's around. She always seems to go back and tells MB petty things about me like "Whitney is in the living room with her feet up doing nothing." and MB will speak up and tell her, "she's been here all day, she can take a break." or she'll suggest that MB leaves me alone with all the kids and run off with her because "she can handle it." In which at the time MB is trying to cook dinner before running off to take care of errands, and I am already up to my neck with chores. She would something's made rude comments, and she's always looking over my shoulder telling me how to do my job. Every time she's around I cringe, because I can't stand the sight of her, and it takes everything in my to bite my tongue and not act unprofessional. How can I address this situation with her and let her know she's being a pill?