This is the same man who, when his biological child came on the scene, had him and his wife draw up wills that left different things and amounts to each child, fully aware that his own DD was going to get more. (OP hasn't said that's what the timeline was, but seeing how old the youngest is and when the wills were drawn up, seems like a fair assumption).
I don't know that the DH has said OP should be fine with her youngest going to private school and the eldest not. She hasn't said. I suspect it won't matter to him decisively one way or the other (and this whole education thing is quite a quandry for OP). I have two friends in so-called blended families who have bio children in private schools, step-kids not. One has a very similar age gap, one has a much larger one so less of an issue. As parents, they both consider that they're working hard to provide for their children, and that their step-DC have two parents who can also choose to work hard to provide for their children (or not) as they please. How can you argue with that?
Nobody is denying that it would be wonderful if all children are treated equally, everywhere. That's just not life: not in nuclear families, not amongst full-cousins, not in blended families, in families in the same street/community/neighbourhood/city/country - anywhere. OP has decided to raise her two DDs at the fault lines of these divisions <shrug>. This is what happens.