Thanks for bringing your question to JustAnswer.
It sounds like you hoped you'd get some siblings-in-law. It makes perfect sense, with you having been an only child, but here's the thing: no one from the outside ever really gets accepted into a sibling group. They may be polite,etc., but you will be left out of certain things that the siblings do or talk about.
I really think this isn't about feelings (or lack of them) that they have toward you. People who grow up as a family are going to have so many "secret handshakes"--certain understandings of what is and isn't discussed, deep knowledge of each other's true natures, etc--that no one who didn't grow up in the group can hope to can ever hope to understand them all.
Because here's the thing...there are certain advantages for you. You get to choose what interactions you will be part of, or not. The siblings are in for in all. And no matter how much fun it looks from the outside, I can almost guarantee you that there are undercurrents between them that are less than pleasant.
As hard as this will be to hear, you will never be a full-fledged member of the sibling group. Part of the problem of interacting with such groups is that they revert, to some extent, to less mature ways of dealing with things when they're all together. This probably is some of what you've felt as rejection and disrespect. Are there spouses of the other siblings you could focus your friendship on? They most likely feel the same exclusion that you do.
Your husband has no idea how it feels to be on the outside of his sibling group, so it's not surprising that he doesn't understand why this is an issue for you. And no matter how siblings actually feel about each other, they will usually defend each other at all costs.
It sounds like you've done a lot of great growth work the past few years. Perhaps you can use those same tools to come to acceptance that your role in the family will never be as a sibling. It wouldn't matter if they were the kindest people in the world--it is just a different kind of relationship. And frankly, the last thing you want is for your husband to see you in that role...it would have a detrimental effect on your intimate relationship.