How old are your kids???
Now: per my kids and some of our icky relatives, I simply taught my kids... that not all people are fair or nice. Even family, can be that way.
But in no way, should they take it personally. Because, it is that relative's "problem."
My kids KNEW... which relatives are that way. They would tell me and I would say YES, he/she is that way...and you are correct. It is selfish. BUT, you are your own person. Don't take it personally. They have a problem and don't know how to behave. It has nothing to do with you. Because, those relatives were like that, even BEFORE you were born.
And its true.
And my kids understand.
And they understand about people.
And that, relatives or not, ANYONE can be mean or not nice or play favorites.
But THEY do not have to take it or take it personally.
They are their own person.
And "gifts" do not mean, that one person is "better" than the other.
I taught my kids these things, since they were Toddlers.
Because, I have some icky relatives, just like that.
And I did too, as a child growing up.
And I KNEW which uncle or Aunty was mean, that way.
And it did not bother, me.
Because, I knew how they were.
And that it was their, problem.
Not mine.
And it is not, nice.
Whenever my kids would tell me of an observation of our relatives... that is icky or unfair in attitude/gesture, they were right. And I would tell them they are right. I did not sugar coat it or pretend about it.