Given your situation, I think you absolutely have to step back. This is his daughter and her life. She knows her father, she is living with him, and you are separated. Please don't get involved with your FIL. There's just no way to make this work. It's falling on deaf ears because, well....all the reasons you are no longer in a working marriage.
If you see your stepdaughter, you can mention to her that she should get free help from the financial aid resources available. She can apply for schools she can afford - "top notch" doesn't always mean "good deal" and it doesn't always mean "most expensive". There's no reason to downplay the value of the many schools that would love to have a smart and motivated student like her, or to feel that she's being shortchanged by not attending, say, an Ivy League school with huge competitive odds. I'm not sure why you think your FIL helping will count as income for her - granted I haven't done this for about 8 years so maybe I'm out of the loop. The main thing is for federal loans to be in HER name so she doesn't have to pay them back at a high level and so they don't revert to her father - he should NOT co-sign on loans, and neither should his father, even if they help her directly to pay her own loans.
Is this your first go-round with colleges? Please be aware that the more expensive schools tend to give more financial aid than the state schools. They have bigger endowments, and they know that hardly anyone can afford them. We've seen many families who have Kid A at a very expensive private school, Kid B at a state school in another state (therefore paying non-resident tuition), and Kid C at a school in their own state. The out-of-pocket for the parents was virtually the same for each kid because the aid varied so much.
I know you care for your stepdaughter and want the best for her. But you just can be in charge of this. I'm sorry.