If she thinks she's super gran and is happy to live in an outdated bubble and risk your dc in the current world then I'd call her on it. Your SIL may well just be too under her thumb or conditioned by her to accept what she says and finds it easier to let her mum run over her wishes than to argue so the fact that she lets her mum do all this stuff doesn't necessarily mean that she agrees with it (and maybe she is being told that you don't care about it so she shouldn't too). She may love you for standing up to her mum about it.
Next time she does something dangerous or you discover she did something dangerous while you weren't there or she isn't prepared to use something safe (eg throwing damaged car seat out and using a new one, stair gate etc), then give her a warning that it's something that is very important and thsu non-negotiable to you and dh, and that if it is a problem for her then no worries, you understand that as she is only his granny and obviously doesn't care that much about his safety/life/etc (as appropriate to thing under discussion) so you can take dc away again if she obviously doesn't care. If she does and makes the changes then he can stay.
Then she has the choice - be safe and have ds or continue on with her outdated ways and unsafe equipment/views and miss out on spending time with him.
If you're nice you can give her a warning to give her a chance to make the changes - but if she doesn't, then fine, she doesn't spend time with the dc.
And if she argues about it - just say that you love your ds too much to risk the potential harm that could come from using xx or doing yy and you are not prepared to take that risk. If she doesn't love your ds enough to make a small change to substantially reduce the risk then she obviously doesn't love your ds enough.
Remember - It's her choice - not yours. You are not doing this to her (ie taking away your ds) - she is pushing him away because she is prepared to continually put him into high risk situations in which he could be seriously hurt. That is not the behaviour of a super gran.
Passive aggressive and mean - yup. But hell, she doesn't care about your or your son by the sound of it, she just wants an easy life and to be able to have your son without taking responsibility for his safety. Being called a bad gran is hardly unreasonable in the circumstances!