Hi @ShellieL I'm glad things have calmed enough to sleep. Gonna add my 10p worth, FWIW. I am part of a blended family with adult DSC as well as my own adult DD. There have been several difficulties over the years, and in the past it has been about the only thing we have argued about.
Adult DSC went through a stage of being extremely needy. Older one in particular was always borrowing and not repaying money to DH, and it was particularly hurtful when she denied she was invited to my hen do 6 yrs ago. This festered rather, and I have approached it, tactfully, this week, in reference to her own hen do, which is coming up.
She has responded well, and much more maturely than she would have in the past, so we seem to have reached a respectful truce. It would have been unthinkable a couple of years ago.
So, I think what I am rather long windedly saying, is that sometimes, in my own case, when they mature, and I stopped looking for any sort of validation from her (I don't think she has ever shown any interest in my fascinatingd life ever) then things became pretty ok.
On the other hand, I know someone whose step children were so hostile, and their father refused to address it, that the relationship ended. Only you and DH are able to work out which of those it is, but preferably when everyone is calm. I know this, as I have the t shirt!
I wish you luck. I know it's hard, but not impossible 