I find people's perception of family sometimes perplexing, but then they may find my approach to family perplexing themselves. So...
Having said that, I understand half of Perforin's point.
The issue is that I find it strange to shell out the cash to the son and neglect your daughter and son in law. If they didn't give their son any money, none of this would be an issue.
I agree that in-laws helping pay for a son's education while leaving a son-in-law out of consideration would be a little... upsetting, I guess, depending on many things (like if they had already paid the daughter's education expenses, etc.)
I know two couples with similar issues in the family: one is a female who married while going to medical school, and her father helped pay for vacation trips, a car, and medical expenses for his daughter. Despite it being almost solely the responsibility of the husband to do this (in their culture) the father helped to make his daughter's life "better".
The other couple is a doctor who married his wife AFTER (or very near to) finishing school and then NOT continuing his residency training. His wife's family helped pay off all his loans because they figured that would also help their daughter. And only with his insistence have they agreed to let them (not just "him") pay back the loans to them, albeit without the interest.
With this in mind, I think this:
If you haven't asked for help, it shouldn't bug you that they don't offer. Have you considered they may not want to step on your toes...perhaps thinking you want to make it on your own?
is a very good point.
On the other hand, my family has helped support my siblings in ways I was not supported. Sometimes not because the offer was not there (like buying me a car as they did with my brothers), but because I refused it. I'm a hard worker and I enjoy my independence without having to rely on anyone to help support me. I feel indebted enough they put up with my rebelliousness and put me through 12 years of school and 4 of undergrad, I certainly don't expect them to help me pay anything now, especially not something like medical school.
So, the OP's scenario wouldn't bother me. In fact, it might bother me if they offered, although I'd never decline anyone offering help to cover tuition for medical school. I'd take it and suck up my pride and live with the burden of feeling like I owe anyone anything.