You have no obligation to leave your child with anyone unless you feel comfortable with it and want to do so. 18 months is still very young and a dangerous age for getting into things - especially in an environment that is not baby proofed under the supervision of people that just don't see the dangers.
My in-laws live in town - one just on the next street over - and we don't send our kids to stay there. If the kids (21 months and just 3 years) are going to be awake the whole time, we'll drop them off at my mother-in-laws. Otherwise we always ask her to come to our house so the kids can go to sleep in their own bed. We even had her come to our house at least for sleeping when she watched our older one when we were in the hospital having the second (the ONLY time we've left either overnight, by the way!). We always use the argument (which is a reality) that they won't go to sleep other places without a lot of work and they don't sleep well. We've had plenty of experiences with this from family trips when we were the ones trying to put them down. My mom lives about 45 minutes away and has to do a lot to help her 88 year old mother with Alz and my neices so we don't ask her to leave to stay at our house. But her house is better equipped to deal with young children because of my neices being there a lot. Again, I think my daughter was only there one of the nights when I was in the hospital. If she watches them, we just pick them up in their jammies and bring them home to sleep otherwise. I don't know how your son does with going to sleep but that could be one excuse.
I know what you mean about just offering things to eat and not really checking to see if things are ok or not. Mine do the same, but have improved over time. We are vegetarians so we are very watchful of their foods. Our families don't offer them meat exactly, but have offered things that are made with animal products other than dairy or egg (we eat those). Things with gelatin is a big one. It's usually just them not knowing. After a time or two of that with things that they didn't imagine would have it, they got better about checking first before offering and having our child lose it when we take the item from them. This summer my younger one had a severe nut reaction so now they are even better because we've made it very clear that he can't have anything related to nuts, processed with them or anything. So we basically make them show us labels now or they can't give them food. Our kids really love fruit so we've gotten family to see that over time and now they will often offer that as a treat which which basically never argue with.
You said your in-laws are very lax on safety. I'd be concerned with leaving him because of any driving too. My husband and I are really particular about our kids and their carseats. I just don't trust really anyone to be as vigilant with buckling. My father-in-law as talked about taking my kids in his car and there is just no way. He's not a safe driver and he's clueless about the carseats. He has a way of not really caring about details and doesn't pay attention. I don't trust that he'd get all of the 5 points attached properly or remember to pull up the chest clip. On that piece we just say it's a big pain to pull carseats in and out - which it is. We've got 2 next to each other and soon we'll have a 3rd across the back in a station wagon. There is rarely a reason it's worth putting them in another car...again when we were in the hospital having our second.
I'd just make a fuss over still being new parents and just blame it on your being over-protectiveness. Tell them you just aren't ready to be separated from your son for several days and not that distance. There was a possibility my husband was going to have a work trip/bonding in the Bahamas that I was also supposed to attend. We had both decided that we would not attend under any circumstance and go that far from our kids - maybe we're crazy, but we like to be close to them and just aren't willing to do it.
Just still to your guns and know that you are right. Maybe you can offer that they could come and stay at your house for a few days with him to see him longer. Or maybe when you're ready, you could ask them to even watch him at your house if you needed to be gone overnight. That way you at least know the environment is baby proofed.