I’ve been on your side all the way and still am, but your latest updates have got me wondering a couple of things.
Does your DH feel like he has equal parental rights over your eldest? If he doesn’t, could he be putting up barriers to protect himself? You may have felt all these years, that as he’s not DD’s biological father, you didn’t feel right expecting him to treat her as such financially, or with decision making, such as schooling. Instead you’ve been happy that he’s treated her equally in everyday life, but tried not to burden him with the bigger parenting stuff. However, HE may feel like you’re holding him at arm’s length somewhat and actually he’d like to have a say in elder DD’s schooling and support her financially, such as in his will, or with private schooling. I think this is worth putting to him, as it may bring you guys back on the same page.
The other question in my mind, is if he has been influenced by his family, when it comes to his thinking about equal treatment, when it should be him teaching them how to behave with his blended family. The fact that MIL has gotten involved to try to resolve the problem and SIL has tried to extend an olive branch by messaging you directly (albeit badly), suggests that they do care about resolving this, but really need DH to be clear about how his DSD should be treated long term to clue them in to the deeper issue. Perhaps they’re on the defensive because they know they’re wrong, but are ashamed to admit it. They need someone to help them fix the problem and it should and can be DH, by telling them what you really need. Fine she won’t inherit from them, but she should be an equal participant in family events, as you are a family of 4. Seems like if DH could be firm about this position, their attitudes may well change for the better.
In this case, I would ask DH to speak to them about the genuine issue and how he’d like them to behave. Show that he loves and will stand by you guys. Be the stronger character. If he agrees to that, you could agree to apologise for reacting so angrily, explain that you love your family and daughters and want everyone to feel equally loved and that you hope they can understand. Also you could thank them for talking about this and working to resolve the issue (you don’t have to forgive them privately, but publicly it could bury the hatchet and allow things to settle).
Of course first, you need to be okay with DH, so tackle the first point, first!
Just a thought!