You have to keep reminding yourself that your health and the health of your children is more important than their feelings. What his siblings do with their own children is irrelavant. What if they decided they didn't want to buckle their children in car seats and seat belts, would it then be ok for your children as well ? What if they decided they weren't going to put helmets on their children when they ride their bicycles ? What if they decided they were going to leave their children in a hot car by themselves for a long time ? Would it be ok for your children ?
Listen, some people are just plain dumb or they are complete pushovers. His siblings and their spouses are probably in that group or they engage in the bad behavior themselves and don't see a problem with it. At a certain point, your inner mama bear has to come out and do what is in the best interest of your baby. Not what is best for your husband, not what is best for his siblings, not what is best for his parents, you have to do what is best for you baby. Again, what they think doesn't matter. It doesn't matter one bit. Speak up for yourself and your baby since your baby can't speak up for themselves. That is your job as it's mother. Yeah, your husband and his family will probably get mad, but so what. They are the ones in the wrong here and you know... this is all part of growing up. Using your best judgement and do what is best for your family.