in my experience it's good to accustom tots to being in the car as they'll end up being good little passengers then as they get older.
'Accustom' is a wonderful euphemism for adults doing something they want to do which they know will overwhelm their child.
Research shows that children learn least under stress so if you want them to be 'accustomed' to the car, build up the journey length slowly, always keeping it to what the child can handle without becoming distressed. Children don't get accustomed to things that they associate with being bewildered, hungry, bored, cramped and overtired.
There are many, many things that babies have to put up with. They have to go places they don't want to go frequently and my baby often wails disconsolately while I'm preparing a meal. Yes, he has to put up with it but it is not consequence-free for him and I have to ensure he doesn't become unreasonably distressed. IMO he spends more than enough time not being the focus of my world - there is no need for me to dream up
excuses for doing things that will cause him distress, exhaustion and a degree of pain in order to give adults a little extra pleasure.
Adults are supposed to be kind to their children and to respect other parents when they choose to put their children's needs ahead of their own desires. Adults are the people who should get a grip and learn to take minor disappointments on the chin. Babies are tiny and have no way of rationalising a routine gone haywire or putting it into perspective with the thought that this colicky, sleepless night will be a thing or the past in a few days.
IMO, relatives don't actually want distressed young children at their gatherings. They don't really know what they're signing up for when they insist on a baby's appearance.
When it comes to family gatherings, I have always said I'll come if I can, meaning I'll come if I can do so without cost to the children. That has always been fine and as much as I'd be sorry to disappoint relatives, they are welcome to visit here so I wouldn't give a fiddler's fart if it wasn't. DH is a big boy and can get himself into the car and down the road to see people if he wants to. Not that he'd bother.