is it possible that his parents are not able to visit back east and the only contact he has with them is if they come out to visit with them. instead of feeling hurt, you should feel proud that you brought up a daughter who is thoughtful enough to put her wishes aside for fairness in seeing all of you without favoritism. once a year with both sets of parents; you when you visit, her in-laws when they travel.
perhaps she didn't tell you when they were there because she was afraid of hurting your feelings.
it is not selfish to want to see her, but maybe she is just trying to balance the free time they have as a family.
it is only natural to want to see her more often.( she probably would like to see you more often), but as you said she is very busy.
if only we had all the time and money in the world...sigh.