You nor your DP can make your dh family, especially his db, see your eldest as part of their family. It’s blindingly obvious they don’t. It’s not going to change. Your mil is not going to get in the middle of her sons.
ignoring the step up comment for one moment, how is DP with her normally? You say he treats her the same ? If he really does, every single day, then I think his anger is at the situation. He also initially thought you were all invited and was unhappy that that isn’t the case but he also loves his brother and wants to be at the wedding.
you know Dp best, is the stepping up comment more of a Ffs I’m caught in the middle of a shitty situation (eldest is his dd for him and not extended family) or do you think it’s more- I wouldn’t stay if eldest isn’t loved too. Try to remember he can’t change his family and he loves them even if they are idiots.
Depending on dh answer, let him and youngest go. Take eldest on a special day out, somewhere you couldn’t take youngest.
tell dh that he’s now in charge of all family interactions on his side. They apparently only see blood relations as family so your out. You’ll be polite but that’s it… he does birthday, Christmas etc but not to detriment of your family unit.
you Are rightly angry op, and if you need Dp to choose your family unit or it’s over then that’s ok but don’t make the choice when you are angry. Its also worth considering if your anger is really at Dp or if you are angry at his family and he is taking the fall out for it