To ask for pithy but classy comeback for SIL?

You’re not looking that bad at all., honestly.

😂 I love this. It's the word "that" that is the real killer!

If she calls herself a heifer you might say: Oh dear you do struggle with body image, don’t you. You’re not looking that bad at all., honestly.

She might not have directly called you fat but if she was calling someone smaller than you fat then it was still an insult to you.

I couldn't have not said something. Like PP's have said, you are letting her get away with it.

I agree with everyone above, Bullies and assholes like your sil get away with it and escalate / feel entitled to comment / become emboldened because no one challenges them. She probably thinks everyone is secretly agreeing with her, not that they all think she's a complete cunt.

I don't mean to be mean, OP but if you're not going to say anything when she repeatedly acts like a a cow to either yourself or others, you shouldn't really be walking away and moaning about her behind her back.

In short, although she's the AH, you've made your bed and covered it with a big fat duvet stuffed full of acceptance.

Either lie in it or speak up.

You've been given good advice here but you've just sat back and let her run rampant unchecked yet again.

This ^^ Just be completely literal in your answers. If she gives you a back handed comment just thank her and agree you’ve been feeling wonderful lately. If she goes on about her weight, how she looked etc, give her appropriate advice, on diet, exercise or fashion. Tell her your sorry she feels that way.

Start by walking in with your head up and smile as you chat to people.

I did this once with my mother! She came to visit and my husband and I took bets on how long it would take for her to mention my weight (she made it to 40 mins). When she did we both looked at our watches and started laughing so of course she asked why so I told her! Shut her right up 😆

I look her in the eye, shake my head and say ‘it must be so hard being you’

Why is that the OP’s job? Presumably the other SIL isn’t incapable of tackling it herself if she wants to.

But you let your other SIL be subject to that and stood by?

How did your other SIL react? It's clearly not just you, surely everyone hates her predictable, snarky comments. Didn't anyone tell her to give it a rest?

I have so many questions 😅 sorry OP, you are definitely better off avoiding indefinitely.

Or yes I look awful too, isn't it terrible that we are both so fat. And laugh heartily.

When someone (related by marriage, didn't see often & I definitely didn't care for) said 'you've really put on weight ' I told him 'I could lose weight but he'd always be a cunt'. & the last I heard he's still leeching off his ex wife's family in their home country 🤮

Not suggesting your sil deserves that particular phrase but pick something that will shut her down and stop her comments going forward.
I love the haircut suggestion, the 'are you tired' suggestion too.

There is zero need to discuss another adults weight with them unless they raise it first!

You could say 'My mother taught me not to discuss religion, politics, money and personal appearances in polite company because it's deemed rude'

Regardless of how anyone feels about what they are doing, they are technically breaking the law by committing indecent exposure and/or outraging public decency. Doesn’t matter if it’s in their own home, it’s about other people being exposed to it. They are obviously doing it on purpose which is pretty grim. If they want to put on a floor show I’m sure they could find a willing audience somewhere instead of inflicting it on their poor neighbours. Tell them they are clearly visible from your children’s bedroom window and if they don’t stop then report them.

Just look into her eyes and say ‘wow, just wow’

I was taught how you to use "bless your heart" by an amazing deep south Lady (yes she was a real Lady!) and its brilliant.

Doesnt have the same power over here as it does there but it does work. Its dismissive qualities are incredible if you say it as you look away, making it clear that what she said has no importance to you at all.

I was also going to suggest the bingo card with an obvious title like “Bastard/Bitchy Bingo” on the top.
Or have some pamphlets for eating disorder advice to give her, citing concern for her preoccupation.

I think the Americans would say “oh bless your heart” in this sort of situation which I think basically means “Fuck you, lady” while also sounding nice, so that would probably cover it…..

I'm such a heifer... Don't put yourself down it's so sad that you let your weight define you, have you tried working on your esteem, you're such much more than just a body, don't value you yourself on that, it's really not that important and shouldn't make you feel so awful about yourself

If someone commented on my weight loss in that situation, I would just say “thanks”.

Because I genuinely wouldn’t care what complex drama they had weaved around it in their own head. I would have no interest in talking about it and the quickest way to shut it all down would be a polite “thanks”.

She says "you look well'. You say "aw thanks, I'm feeling great!"
She says "Im such a heifer'. You say " you're too hard on yourself"
Ignore the game playing. Be generous and confident. You'll feel.much better than if you hand out some smart remark.

When she tries to do the self-deprecating “god I feel like such a heifer at the moment” thing, agree with her. “Ooh, yes - I was just thinking you look a bit chunkier than normal”

Something which suggests that you’ve noted it is frequent.

“I was wondering how long it would take you to comment on it.”

“Yes, I expected you to notice.”

”I did think you’d mention this and you’ve not disappointed me!”

"oh I look like a heifer!"

"Well I didnt like to say, but have you tried low carbing?"