I always feel like a coward for not challenging SIL

Totally agree.

What an awful way to talk about other women.

But nothing it says, speaks well to OP.
you might think you’re doing haughty disdain like some dowager, but you seem to have forgotten that silence is also assent. OP assents to those comments around her. She is teaching those children exactly the same message as her SIL is.

But in this case, the SIL wasn’t even talking to the OP, or at her, she was addressing her own teenage daughter about something specifically she saw as relevant to her. The OP could look as blank as she wanted because no response was required.

She’s different from you. So what? You sound quite judgemental.

I'm waiting to hear what she did/does that is so terrible. If she triggers you so much, just stay away from her. With the teen mum thing, she was talking to her daughter. Presumably she knows the best way to communicate with her - when you have your kids, you can educate them as you see fit.

Sometimes silence is golden. People like this often want either a debate, or for others to join in and agree with them, so they can warm to their theme. They can be made to feel very uncomfortable and insecure by having someone simply look at them blankly and not respond. I think it's quite a powerful weapon actually. Done well it says more than words ever could.

She's entitled to her opinion; you don't get to tell her to shut her mouth. If you disagree,argue your point and show her she's wrong

Honestly, why do you care!?

you weren’t a teen mum were you? So who cares

roll your eyes and ignore

your sil and her dd are not your responsibility, just drop the rope

Yeah, but as you say you’re a coward. I’m not sure cowardice is any more preferable as a character defect.

You let your neighbour be rubbished in front of you, because you have no backbone to say what you think. You couldn’t even manage “Oh that’s a bit harsh, X over the road is doing her level best” or “yikes! I hope you don’t say that about me when I’m not here” or any number of things that mean you actually stand up for your values.

You have been polishing your halo that she talks shit. But you’re no better.

Appearance isn’t an approach I would take by default. However, you have to play to your audience. It’s possible that she knows her daughter isn’t going to listen to arguments about financial stability and being able to escape bad relationships. If looks is the way to get the message across to this particular girl who has recently become fertile, then she should use whatever means are necessary.

I could accept an aunt saying that it wasn’t ok to disparage appearance. If you argued that teenage pregnancy wasn’t catastrophic in front of my teenage daughter, that would be the end of our family gatherings.

Surely you could just burst out laughing when she says things like 'women who have children young age in dog years'. For starters there's no such fucking thing as 'dog years'. She sounds like a misogynistic twat which is quite an achievement for a woman.
Just don't engage with her unless to roll your eyes in an extremely obvious fashion.

That is the whole point of having an opinion. People don't think that their opinion is wrong, otherwise they would have a different one ...

People are allowed to express their opinions in their own homes, even if other people disagree.

I agree with her that teenage pregnancy is less than ideal, and I think the potential effect on your body is not the worst place to start with that age group. Attention-grabbing if nothing else.

Meh. I definitely feel like I aged in dog years when DCs were little.

Now they've grown up I'm off travelling the world.

Shes right.

Telling her to "shut her bloody mouth" would be far ruder than what your SIL said.
If you can't express your own opinion in a calm and polite way then it's you that has a problem unfortunately.

You seem rude and obnoxious @Hour.

SIL and brother are quite odd in that their whole family does just end up sitting in their kitchen doing nothing but talking about hypotheticals. I guess it’s good because they are a chatty family but it does mean so much shit is spoken

Instead of being rude and obnoxious, you could calmly make your own points.

Or better still, remove yourself from their kitchen.

If you don't care, why do you care?

About whether it's 'cowardly' not to challenge someone you strongly disagree with, I think it depends on the context. It might be cowardly not to speak up if SIL was abusing someone to their face and the other person needed support, but the examples you give are not quite like that. What are you not saying that you'd like to say - is it just 'God, you are so opinionated' or something about the subjects under discussion?

meh. Challenge don't challenge.....why do you bother to sit there?

She's your sister in law, you didn't choose her. She's not your cup of tea. Limit the amount of time you see her and just exchange pleasantries. It doesn't mean you are weak .

But everyone thinks their opinions are the right ones. Otherwise they wouldn't hold them. Why would any reasonably well-informed, well-adjusted adult hold opinions they didn't think were the right ones? And share them with their own teenager in your presence?

Doesn’t everyone google celebrities and then possibly slag them off and say ‘blimey she’s aged, or god she’s a bit ropey for 42. Similarly you will say ‘she looks fantastic!’ That’s not being nasty, well it would be if the celebrity heard but it’s unlikely.

I agree with the dog years comment and think it’s funny.

My in-laws are the world’s most boring, opinionated, mute family. It’s painfully dull when we meet up, so your SIL sounds like she livens things up. If you genuinely don’t care then carry on saying nothing, if you think she’s talking shit, challenge her and say ‘well that’s not strictly true because I believe XY and Z.

Can you just say Oi SIL you don't half talk a load of old shite ? 😀

This isn't rude though. So if that is an example then YABU.

@IdgieThreadgoodeIsMyHeroine "I've invented a new drinking game: every time a Mumsnet user slags off Mumsnet users, take a drink"

And when you get bored with that one, try taking a drink every time a Mumsnet user automatically supports another Mumsnet user because she slags off an in-law.