I'm going to disagree with you OP. You often say that you doubt yourself, you worry you are over reacting and are not sure if you are correct.
But stop doubting yourself! As the thread goes on you do seem to be much clearer about what treatment you have received, what you want, what you don't want and that their actions towards you are unkind. So I think you should take comfort in the fact that you are clarifying your situation even if you don't yet see exactly how to achieve an improvement yet.
I think its very normal to walk into a situation in a very optimistic way, open, friendly wanting to build good relationships. Needing to... and it turns out that you have been continually rebuffed in a hurtful way. Added to a difficult past, which make you more vulnerable to rebuffs.
Stop doubting yourself. Your opinion, your intuition is as good as anyone else's. Its based on a series of things, not just a one off and its affecting an important part of your life, your future.
I think these feelings are overwhelming you now because this "holiday" has really opened your eyes and confirmed that you are not imagining it.. and the feeling welling up is why? Why don't they like me? What's wrong with me? Why can't they just be nice? What have I done wrong?
However, I don't think they have even made a conscious decision to do this. They are just not people who see it as their role to be kind to others. They have formed a strong clique. SIL joined the clique by adopting their rules as her own. They are full of themselves and do this by putting others down. They've probably grown up in situations where there's a scapegoat and if there's anyone with a chink in their armour they go for it. I'm betting your DH was previous scapegoat. Bullies basically. Its literally not YOU. its them.
It would really help you if there was a real life person you could talk to about this and get some techniques for dealing with it. Its great to have friends but it can be too much. You really deserve to see someone who can help you through this professionally. If they were colleagues, or neighbours, or school mums the answers would be simple. But its family, there's a tradition of the way things "should" work, there's guilt about stepping back on DH's behalf. And you could do with some help now to sort this out in a way that both you and DH can live with.
People have suggested stepping back, getting far less involved with them, grey rock etc...I agree. No need to make big announcements, just quietly do it. Decide to spend more time around people who treat you with basic human courtesy , lots of time with people who are nice and make you feel good and as little as possible around bullies who make you feel like shit whatever their supposed traditional role in your life is supposed to be.
They have not turned out to be the family you hoped for, so now you can start building your own "village". Be busy. Make your own plans and events and filling your own calendar with things to do with your own little family its one of the best ways to do this. Weekend clubs are great for this FYI