Okay I know I will be the opposite here. My take is this, and this is coming from someone who went through 7 months ( my entire engagement) of dealing with my DH awful family, the stuff they said and did to me and my family was horrid and what they finally did to us at our wedding ( MIL went after me at my own RD) That being said, when all of it started, I tried really hard not to make him pick and tried harder to make whatever crazy go away, and it didn’t ,things got worse and worse. NO ONE should ever have to put through that, my DH was there for me, he was on my side, he understood what they were doing, and he tried to stop it, for 5 months, but he never cut them out of our lives until 2 months before our wedding, even though they were toxic and destroying me, us, and him. I couldn’t function, sleep, eat, or even able to enjoy my engagement or be happy anymore because everyday was a bigger mess with his family (and they lived 7 hours from us). I understood this was his family but I also understood that no one should be treated this way, there comes a point in time that the person you’re marrying has to pick a side if it crosses the line of disrespect and cruelty towards that person. Just because it’s family doesn’t give them a free pass to treat someone that way. I love my family very much but I would never stand for anyone to disrespect or treat my loved one in such a manor. It took my DH a long time to see what they were doing, and thankfully his families behavior at the RD didn’t destroy our wedding day because we had to keep telling ourselves we wouldn’t let them ruin this as well, but it was still a very emotional day with bad memories of them and the things they did. You can never take those away, and we both loved our wedding day, but what they did to us will always hang over our head. My DH regrets not realizing and cutting them out of his life sooner and letting them destroy a time in our lives that was supposed to be the most important and special.
With all that being said, I think this only applies, if you are being treated poorly by his family. You didn’t go into much detail, from what I read, you didn’t mention any wrong doing of the family, just that they are different, and that is no reason not to like them. Before all this started with my DH mother and sister, I knew they were different, knew we would never be best friends, but still welcomed them in my life, because they didn’t do anything to me at that point. You don’t have to love them and be crazy about them, but if they didn’t OUTRIGHT do anything to you, then I don’t see what the problem is.
For the sister, she sounds nuts, but again, has she outwardly done mean things to you? If she has, then 100% I feel that your DH should be taking a stand, but if the issues are with HIM and HER and have NOTHING to do with you, then it’s up to him to pick.
The compromise IMO from what I read is that you don’t overly have to love them but if they want to come visit or whatever the case may be, you need to be welcoming to them for the time being. If there is more to the story and there have been some big deal breakers for you with his family then it’s def something you and your DH have to see eye to eye on.