We are very lucky to own a motorhome. It's left hand drive and we generally use it to travel in Europe during the school holidays with our two youngest DC aged 9 and 7. The motorhome has a seatbelt for the driver, passenger and two seatbelts in the back which are used for the children. There are additional places where you can sit, but no seatbelts.
My PIL have asked if they can come with us on our summer trip to France and drop them with their friends, we would then carry on down through France, into Italy etc. We would then potentially collect them on the way back. PIL are only in their late 70's, however FIL is quite infirm at the moment and not able to make this trip by any other means.
My DP has already agreed to this and I'm not happy. PIL seem to think it's ok just to sit in the back of the motorhome with no seatbelts. PIL are of an age where they have a piss poor attitude to car safety, there is history here. They thought it would be ok to hold newborn DD in the back of the car instead of using the car seat. When older, DD told me that they hadn't strapped DS into his seat and had driven through the city centre. I haven't let them take the DC out in the car unsupervised after this as they genuinely do not see the issue as all is well that ends well. FIL also drinks and drives and MIL just rolls her eyes.
AIBU for refusing to take PIL? I'm not having two 10st unsecured adults sitting in the back of my motorhome with my DC? DP thinks I'm being over the top and this in turn worries me about his attitude to car safety and what happens when I'm not there. DP thinks I just don't like his parents (I'm very fond of MIL but FIL irritates me hugely) and I'm looking for reasons not to take them. I've suggested that they look into the expense of having two additional 3 point harnesses fitted as a middle ground. I don't see what else I can do!!