Sorry this is long!
FIL had an injury which means he could not drive for a while, and as it's 4 hour round trip, DH has been taking a lot of time off to stay there, do the gardening and other chores, drive them around etc as MIL has not driven for a few years and now refuses to. He always offers to take her for driving practice but she won't. They have even been known to ask him to drive the 2 hours each way just to pick them up and drive them 30 minutes to a routine hospital appointment, check up, optician appointment etc. They are very wealthy, and when relatives visit them, they have no problem ordering taxis to pick them up at the airport 45 miles away. Funny though that they can't use taxis themselves, not even for the 1 mile to town, relying on neighbours instead. As they know, their request to DH is taken as a command and will NEVER be refused. I gave them details of their local hospital transport service (the type I use myself as I am disabled and DH has limited holiday allowance), but that was ignored.
In addition he or we have always visited at least once a month, Skyping and calling once or twice a week.
DH has a sister who lives abroad and visits once a year, paid for by PILs. Last week she came to visit, FIL paying as usual. Her new partner also came, (no doubt also paid for), and FIL wanted us to be there too and go out for dinner together. We had stayed there one night and were due to stay a second night. PILs asked us to leave and find a hotel as SIL and partner would be needing the guest room. When we visited the house next day, SIL and partner were in t shirts and the house was even warmer than usual. I appreciate they live in a hot country but FFS, have some common sense! I was wearing light clothes as PILS house is always very hot, and I can't stand being too hot, as they well know. Went to bathroom, cold water on face etc to cool me. Back to sitting room and the gas fire was now blasting away too. I expressed surprise and MIL said 'they were cold' meaning SIL/partner. I sat fanning myself, pouring with sweat. Even DH was sweating and uncomfortable. PILs looked at me as if I was from another planet. After a few minutes I said I'd have to go outside to cool down as I could not bear the heat. I went out to thunderous silence, and sat in the car reading a book. I stayed there about 20 minutes until DH came out to inform me that they had now turned the gas fire off again as SIL was now 'warm enough' 
, still in t shirt btw.
So, if you have read that, thanks, and my question is - am I wasting my time by going there ever again? It's been 20 years, and I have tried to be a dutiful and considerate DIL. Is it time to give up the pretence? I don't get it, that after 20 years I merit no consideration, and am made to feel like a weirdo who makes things awkward. Do they want me to sit there and pass out? God forbid they have to put on a jumper and compromise on heating levels for an hour or two (as we do when they visit us ). I feel for DH who does most of the routine visiting and helping out, but that is seen as expected, but when SIL goes once a year, it's like royalty is visiting.
I am mightily pissed off as it's so unfair to DH really, and then when I go too it's as if I'm some random person they don't know . I feel like never bothering again, but rather let them all get on with it, DH too and more fool him.
I also don't understand why it is seen as incomprehensible if a person is too hot, but perfectly normal and to be sympathised with if too cold? Easier to put another layer on, but there's a limit to what you take off and remain decent.