SIL playing favorites

My SIL has always put her family first, always! It’s starting to really bother my husband and I, because we feel our son is being excluded. So her family is pretty big and her kids have over 10 cousins on her side. My son only has her kids for cousins. She is constantly putting all of them first for holidays and inviting them all on vacation leaving my son out. Am I wrong to feel a little upset? He’s too little to understand but I know he will eventually. I don’t want him to grow up feeling like the odd man out.