I think OP needs a bit of a reality check. She can stay in this situation, or leave.
If she stays: Eldest DD has no paternal family, actual dad doesn't want to know. DH provides for and helps raise her eldest, but it does seem that the day to day household basics will not extend to big financial privilege such as private education nor his own families inheritance. Many women who post on the step parenting forum are told "butt out, the child has two parents and you are neither" whereas here, it seems DH is the devil for already doing way more for this child than any other father figure does, despite having zero obligation too.
If she leaves: Eldest DD still has no paternal family. And now no DH to play the role he was. Youngest DD will continue to benefit from her loving paternal family, their support and money and there's nothing OP can do to stop that, as she is not the authority over DH as she seems to think she is. Equal parents.
So, OP what's better. No father figure at all and a single parent to your eldest, 50/50 (a guess) custody of your youngest who will remain with the privileges of her paternal family. Or a basic father figure to your eldest, full custody of your youngest who will remain with the privileges of her paternal family.
And please take ownership of your life choices OP. You had two children, one with a completely absent and interested father, then another with a present very interested (and solvent) father. Turns out he won't provide hundreds of thousands of pounds of school fees and inheritance to a child that's not his. He will provide a home and her other needs. You are completely unreasonable to expect the former from him. You have zero acknowledgement that he's already doing a country mile better than her actual father, despite having absolutely no obligation too, you seem to entirely gloss over that, you genuinely think it's yours and your DDs entitlement.