I don't get why people on MN think that just because their parents were unusually ahead of their time, that outdated attitudes are "not a generational thing". Being a generational thing doesn't mean every single person between X and Y age believed in something and still does. It just means that it's likely that at the time they were doing their parenting they were following the norms of the time and it's entirely possible that they simply don't realise things have changed.
I don't see why people are so defensive or disbelieving of that? Why would they be looking up what the current car seat guidelines are? It was only really big in the news in 2005 or thereabouts when it suddenly became law for children aged 3-12 to use them. Since they probably didn't have either children or grandchildren in that age group at the time they probably paid no notice.
Besides a lot of parents even in this generation seem to see car seats as being a handy way to stop the child rolling around the car, rather than the child-sized version of a seatbelt for protection in a crash.
Use of car seats for under-3s was only made mandatory in 1993, and I'm not sure that much was made of it at the time, not the level of publicity of "Clunk click every trip", anyway. So if you're over 30 and were already 3 in 1993, it's highly likely your parents never had to use a car seat for you. If you're under 30 but the youngest sibling, they may also have simply continued to do what they did with the older ones. Most people don't go and research new guidance with their subsequent children - they do what they did before unless they have come across new information in the meantime.
Just because a lot of people were using car seats before the 90s, that doesn't mean it was as culturally accepted as it is today. Today most people would consider taking a child without a child seat to be as irresponsible as letting them run in the road, or travelling without a seatbelt. But in the 90s, I can definitely remember a theme, certainly for older children of being squished up, 2-3 to a seat (and seatbelt), or on an adult's lap, sharing their seatbelt. We didn't really think in terms of the physics of a crash. It was just accepted that being somehow partly in a seatbelt was OK. And honestly? I wouldn't be surprised to hear that somebody now in their 60s or so has not moved on from that mindset. That's what "a generational thing" means to me - not that it would be surprising/rare to come across somebody of that age who has kept up to date, but just an understanding that it was different in their day (even though physics and the actual risk obviously haven't changed). Particularly for things like car seats and sleep guidance, where in most cases, we're talking potential harm rather than immediate harm.
It's difficult when you update an older relative to something they weren't aware of and their reaction is defensive/it was perfectly safe, but I think this tends to be a reflex sort of reaction, and I do think it's helpful to communication in general if you try to start from a position of understanding that their actions do not mean that they don't care about her safety or that they meant to do harm. Give them the benefit of the doubt first (I appreciate at the moment you discover something like this, emotions are running high - if there's already been an argument, let things simmer down before you re-approach them) and then approach from the angle of Things have changed now. I know it was different in your day. This is really important to us. It can also feel less personal if you frame it in terms of things like: This is what doctors advise now for cot death prevention or You'll get fined and points if police see you with her on your lap. That takes the focus outside of "You don't care about my child's wellbeing!" which is a very emotional and personal statement which feels like an attack. And when people feel attacked AND feel they were in the right to begin with or can't immediately see anything they have done wrong, they tend not to back down and say oh, I'm so sorry I've upset you, please tell me how I was wrong. Most people just aren't wired to react that way.
Once the dust has settled, and you can have a conversation along the lines of "These are the regulations now and it's really important to us that you stick to them every time" then it may be that you can get your point of view across to them and they will understand that her being on a lap under a seatbelt is not a substitute for a car seat and that in future if they face that situation, they need to either call you to bring the car seat or not go out in the car. And they may well stick to it.
OTOH, If once emotions have cleared and you've tried discussing it from the sideways, not so blameful angle but they still insist "Well car seats are a load of modern rubbish" or "It's just too complicated, it's easier to do it this way" or they have history of acting in underhand or sneaky ways to "get around" other requests you've made, then I would be extremely cautious and yes restrict visits to times you're actually there with her so they don't have any chance to take her in the car without you.