@KeenHiker
He maintains that he loves her but can’t make others treat her the same.
OP you’ve said it here yourself. You can’t force people.
They sound like they weren’t aware of all the circumstances regarding your daughter, and why would they be?? They have a big family, a lot going on and the brother getting married has no kids so is likely oblivious to most things relative to child rearing.
The wedding is about the couple getting married, they are spending a lot of money for their day and are focused on that. They aren’t setting out to get married to purposely target and exclude your daughter to make some sort of point… they have had the mistake drawn to their attention and have rectified it. You have what you wanted - go to the wedding and enjoy yourselves. Be happy for them. It is about them, not you.
Regards your younger daughter being in a special photo, receiving an inheritance, being sent to private school, having special time with her Grandma…. She should be allowed all of these lovely things and not punished because she has a half sister who is less fortunate.
Please don’t deprive the younger child of nice things and opportunities. Try and facilitate a better relationship or advantages for your elder daughter with her paternal family and foster positive relations with the girls.
You really can’t expect your husband to foot massive amounts of money relative to your elder girl or to deprive his own daughter which are the black and white choices you’re laying out for him.
He clearly loves, cares and provides for your elder daughter. But he is her step father not her father.