To give parenting advice to SIL

In both cases you were responding to an immediate risk to a child's life - that is more important than being polite.

Nah she’s not gonna thank you but it needed to be said. See if DH can have a chat with DBIL as others have said.

alternatively raise a safeguarding concern with SS but I guarantee she will know where it’s come from and the shit will hit the fan

So you’ve written him off as not being worth bringing it up with, yet solely focusing your blame and judgement on her. Why?

They sadly sound like exactly the type of people who should never become parents.

I feel your frustration OP. You've done the right thing, but these idiots are not open to being educated. I feel for your poor niece.

For someone you don’t hang out with you’ve kindly given an awful lot of advice. She probably feels every time she sees you that you criticise and judge her.

Maybe the car seat was forward facing for the photo. Could have sorted the straps before driving. Nobody likes unwelcome parenting advice.

It's frustrating and worrying, and of course none of your points are wrong. But unsolicited advice, however kindly you think you've delivered it, is rarely well received.

You tried. You can't do more than that. You do sound a little PFB-perfect which is grating at times. Perhaps going forward it would be better for your husband to pass on the info to his brother. Just because he doesn't help parent his child doesn't mean he won't pass on the info.

It's her niece who is being put in danger because her sil wants to do her own thing! Why would she not be?

I 💯 agree with you. You are right to advice her but perhaps advice BIL. It is his daughter and as a first time father he may not understand the risks.

Whatever you do, it'll not go well. Say nothing, and you're failing to protect your DN. Point out the issues, and SIL will fall out with you. You were between a rock and a hard place: personally I'd like to think I'd have "interfered" like you. Perhaps wait a bit then make some sort of amends, just to avoid further falling out.

Because shes afraid her niece might die of suffocation?

All the things that you say are fact are actually guidelines (which for the avoidance of doubt I personally agree with). You know, I’m sure you do, than not following them changes the statistics for serious accidents/death, but doesn’t result in them. So, your SIL and BIL have made their choices about those statistics. Maybe they’re okay with a 2% chance rather than a 0.1% (20 times higher) chance. YABU. And if you think she hasn’t thought about it that deeply or is just stupid (which is what you come across as thinking she is, tbh) there are ways and means of getting the message across that wouldn’t have resulted in the reaction you received. YABU x2.

“I just care for my child’s safety” 🙄

I feel your pain. I see friends of mine sharing pics of their kids in car seats in big puffy car seats or same as you said the straps hanging off them so they'd be no use in a crash.

It amazes me that people either just blatantly ignore or are wilfully ignorant of basic safety advice for babies especially.

Maybe she genuinely doesn't realise some of these safety points, not sure you could now after this but you could send her links to information so she could read it herself. Sometimes even if she was annoyed at you sending it, if she read it in black and white it might get through?

At the end of the day she's the parent so you can't do much more than maybe make the information available to her.

People like her are why you see these public safety statements that you might think are common sense but a lot of people don't realise.

It’s tricky - both of those are quite serious safety issues so I can definitely see that it’s hard to not say something. It’s not you being precious and criticising her over nothing, or talking about your own personal preferences.

But also it sounds like she doesn’t give a shit.

ETA - but your niece has two parents, you should probably speak to your brother.

I would have done the same as you op as both of these issues are concerning and about safety and as mentioned the car seat is illegal. She sounds lacking in intelligence if she can’t understand that just because her son didnt suffocate, she still shouldn’t behave in a risky manner, especially if her baby was premature. Unfortunately there is little that can be done but I would try to maybe get your dh to talk to his brother rather than speak to this woman.

Completely agree - i will never understand why people put their children at risk. She’s not interested you have tried id leave her to it and keep your fingers crossed

I do agree with you OP and I think in these situations I’d have had to say something too. What does your BIL say about it?

YABU. Unsolicited parenting advice, especially from someone who has a PFB glow to their writing, so is likely to be the same in real life, is very unlikely to go down well.

Just remember that she knows all of this and has made her own decision, rightly or wrongly, to disregard it.

OP I've got no idea what is wrong with the people on here posting as if you're in the wrong or weirdly nosy. You're absolutely doing the right thing to point this stuff out as it's quite literally a child's life at stake. I agree that it may be best to try to get your husband to have a serious talk with his brother. But if she's still using an inappropriate car seat I think notifying social services would be the right move although I'm sure that SIL wouldn't thank you for it.

You are not wrong and I think it’s coming from a place of kindness, you care that the baby is well looked after

but she is not interested so I would stop and let her crack on how she likes. If I was seriously concerned I would speak to dh and ask him to speak to his brother