How to educate in-laws?

Where to begin?
I'll start with my BIL. Single dad with joint custody. I have no clue how my nephew rides with his mum, but I imagine it's the same as with dad. My nephew is 3 (turns 4 April 1) roughly 35lbs and 40" tall and has been riding in a no back booster since Christmas. I cringe when I seen BIL pull up with nephew riding in the front seat without booster or seat belt. He is not only putting my nephew is danger, he is breaking the law. I have no clue how to approach the issue with him, DH says not my kid not my business His seat before the booster was a bought used OHS that is expired. Now that OHS is sitting in my in-laws garage awaiting destruction by me, despite DH saying it's not my seat so I have no right to destroy it.

DH and I are clearly on different pages when it comes to seat safety and thinks I'm nuts for keeping DS1 rear facing and plan to keep him harnessed for as long as possible.

My in-laws, mainly FIL, are awaiting me to turn DS1 forward facing so they can pick him up every now and then. They've beared with me this long but think I'm over doing it with ERF They have a built in harness in their Chevy Venture(not sure what year) bought used and the dreaded OHS in their garage. I doubt that they would properly adjust the harness to fit DS1. So now I'm toying(money is an issue) with the idea of buying a seat just for them to use, but would they install it properly? Doubt it and it wouldn't stay installed.
How do I educate them without seeming pushy and rude?

I know it's hard to approach these things, but I would feel obligated to say something. It's one thing if the child was able to make a decision regarding their own safety, but in this case their safety lies in the hands of the adult caring for them. I would be completely honest with your BIL and tell them how you feel. Start off by being blatantly honest and saying that you were nervous about saying anything, because you don't want to offend him, but that your nephew should really be in a car seat. Tell him that you are concerned for your nephew's safety, and don't bring up "the law" or anything unless it comes down to it.

I'd just be blatantly honest with BiL in a very no-nonsense tone of voice, but also a non-critical tone. And then let him know that questions are welcome before moving on to another subject.

As for your child riding in other vehicles, just flat out tell them that the only way they can transport him is in a proper child restraint. If they're not willing to buy one and you actually want your child to go in their vehicle, start saving for a spare. Otherwise, they can save up and buy a seat you approve.

(I'm kinda glad we had the first child in in our generation on both sides. I've had plenty of time to work on my brother and BiL in a calm conversational manner, should they ever decide to procreate.)

I know how you feel. My DH's cousin has her 4 month old in a SR that I am pretty sure is expired (I think it was her 7yo DD's), with a bundle me and the straps way above her shoulders. DH won't let me say anything because he says it's none of my business and he doesn't want me to cause drama. He is totally supportive of ERF with our DD and correct coat wear ing and everything but he says it's not our kid so we can't say anything.

I realize how hard it can be to see a child you care about riding unsafely, and I DO think it bears mentioning; however, it can be very easy to go overboard and alienate the person you're trying to educate.

I find, when talking to friends/family (who are easily offended), that sometimes it's best to start with something about the difference between "parenting advice" and "safety advice" and let them know I'm coming from a place of love and concern. Then you can hit at it with the facts of why their child is in danger, and what they can/should do differently. If you're new to CPS the "I just found out and wish someone had told me sooner" speech can soften the blow.

Truly, most parents just don't *know better* and it can be really hard to get accurate car safety info. Figuring out the difference between the "people who just don't care," and those who "would do better, if they only knew better," will help a lot when confronting someone about this stuff. But when it doubt, always assume the best! People who totally seem to just "not care" will often surprise you! But beware, once you put someone on the offensive, it's hard to get them to listen to anything, IME.

Oh, and it's usually better to be prepared and lay it all out at once, then leave them alone about it unless they bring the topic up again. If you keep harping on the issue, bringing it up repeatedly, you'll just push them away and become "that crazy car seat lady" that they always ignore. If you generally email these people, sending links works well, but try not to overdo it. Give 1 or 2 good links, and keep what you say to a minimum. Or if you see them in person often, have a (credible) printout or two. Some people just need to hear it from a different source to really *hear* it, KWIM?

But ALWAYS feel free to always insist on best practices for your child, and any child in your vehicle. I don't think it's unreasonable to insist that your child's grandparents learn how to correctly use a RFing safety seat. If you can fit it into the budget, a $40 Scenera makes a great "grandparent's seat." Give them the choice to having your install it every time they want to pick up DS, or teaching them how to install it (then check it when they pick him up). If they balk, remind them that a LOT has changed in car safety (not just for children, but them especially), in the last 20 years. Everything you are insisting on is solidly researched and evidenced based, and you'd be happy to print some info for them if they're interested in learning more (or websites for them to check out if they're web savvy). "Of course, I'd LOVE for you to get special time with DS... but, uh, duh, you want make sure your grandchild is SAFE right?!?"