I don't want to pay for my stepkid's education despite paying for my own kid
Why don't you like your stepchild?
It's not because of them. I don't want to pay the $30,000.
Education can certainly be expensive! Showing such favoritism towards your own kid can be very hurtful towards your stepchildren who probably consider you a parent. Is it possible to help all of them on a smaller scale?
They are going to get their share of my inheritance and more.
Even still, I think you should consider helping your stepchild out with school as well. You ought to consider them part of your family, biological or not.