I am amazed (and not in a good way) that your parents would ignore one of your children and shower the other with gifts. Is this culturally based? Or just quirks unique to your parents? If this was just unique to your parents (not some huge cultural conditioning), if I were in your shoes I would probably say something like, "It is important that our children be treated with equal respect and kindness, especially by family! That you choose to ignore our daughter and favor our son is disturbing."
I'm not sure how I would go about it from there... I'd definitely discuss it with my husband, but we'd probably be throwing around ideas like:
* really limiting the time the children are around them (like to a very, very small amount and not on birthdays)
* requiring that any gifts for your children be given to you first before being distributed, and make it clear that equal recognition is required - so, no gifts for daughter from them? well, no gifts for son from them, either (and that's not meant in a petulant way, but personally, I wouldn't *want* any gifts like that in my home - they're icky when given under such circumstances of blatant favoritism)
* any monetary gifts from them, simply split in 1/2 and give equally to each child (your parents might not do that, but you certainly can)
I don't know... it just seems so... incredibly unusual and unkind to play favorites between grandchildren. And *NOT* a behavior I'd want modeled for my children. I'm sorry you're having to go through this. I hope you are able to come to a point where this is no longer an issue.