I think it's you being weird.

I am a 24 year old female and am very similar with my 23 year old brother. We've both moved out of put parents house, but whenever we are back for any reason we generally have sleep overs in the same bed. I regularly invite him on holidays with me, and chosen to go on holidays with just him instead of with my previous SOs. I invite him to basically all social events I go to.

And none of my previous partners have had any issue with any of this. They have all made an effort to get as close to him as I am. He has remained good friends with some of my exes.

You're almost implying there is something incestuous going on here. I can almost assure you there isn't. I know lots of girls with who are close to their sisters and think nothing of sleeping in the same bed as eachother. I can't really see why it should be any different because I have a brother instead.

You're unfortunate that you haven't been able to experience a close sibling relationship. There is no other relationship I have in my life that is even close to the bond I have with my brother. It's like having a best friend that has experienced all the same things you have, and you know for certain they will be around forever - mostly because they dont have a choice.

If you give him an ultimatum between you and his sister, he is going to pick his sister - and rightfully so.

I understand it might seem a bit weird to some - I'm happy to answer any questions you might have as someone who might have some perspective on how your SO and his sister feel. But whatever you do, don't try and force them apart - he will resent you for it forever.