Divorce Support Forums: A question for stepparents

I wrestle with the "equal" issue all the time. Got a stepson 16 living with us full time, and 2 small ones (3 & 4) that are ours. Stepson has had a roller coaster these past few years. His mom was married to a man who made a lot of money for about 5 years. They are recently divorced. Beloved (Ha!) Stepdad has not attempted to contact son since mom moved out. Kid has seen his mom twice in 18 months. She is unemployed and very ill, mentally & physically. We have heard she is living off the charity of an old friend at this point. Any way, here is where I wrestle with "equality." Stepkid lived in the lap of luxe for 5+ years where the word "no" was never uttered. Both this kid and his older brother have been told that, of course, they would have private college wherever they wanted, since they were young. Whenever their dad tried to "butt in" with his two cents, he was told that his opinions were not needed. And while stepdad was playing "daddy," they even told my husband his money wouldn't be needed. Well, now one is in college, the other will go in a year, and we are footing 100% of the bill. We sent older one to private school for 1st year, and told him that he had that year to wrangle himself some grants/ scholarships etc. He has not done so, so we are forcing him to transfer. I'm sad, because he did well there, but we're in that financial spot where we can't afford the tuition, but make too much to qualify for aid. I feel particularly bad, because if his dad & I were not married, he would get fin aid, b/c dad makes no money. Anyway, back to my bio kids. My mother has set up educational funds for them. I am also saving vigorously for them. (Note - I saved for my stepkids, too, but I was the only one out of all four parents saving, and I wasn't in their lives long enough for it to add up to more than $25k or so for each). Education has always been extremely important to my family. (It's important to my husband, too, but he's a little late to this party where his older boys are concerned) So, of course my own kids will get what is being denied my stepkids. Stepkids already know about this money, as Grandma is also paying their private primary school tuition (lucky me!). My mother has never offered to help with my stepkids education. They have a good relationship with her, but she does not "approve" of them, because although they are basically good kids, they are not exceptional students, so private school would be wasted on them. I don't feel this way, but just don't have the money. For my own kids, I'm making sure I'll have the money (including Grandma's contribution)

My mother has strange relationships with my half-siblings (her stepkids), and I've tried so hard to keep that ickiness out of my family. For instance, my mother has paid for her step-grandkids to go to college. But, unbeknonst to my half-siblings, she has paid for it out of the trust fund she manages that was to be my siblings' inheritance.
Any thoughts on how we can keep things "equal" in this scenario? My half-siblings resent me tremendously because I got the Ivy college while they went State U. (They conveniently forget that I had straight A's while they had straight C's) I don't want to create that in my current family. But it seems plus ca change....