**Edit: Just e-mail her... and ASK her, if only the girl is getting a gift?
And if so, WHY?
And, I would not make 'excuses' for her, to your son.
Explain to him that some adults are just not nice or fair. That he is a great kid/ you love him, and that, presents don't make, a person.
Or get him an extra present.
Since this Aunty might be, preferential.
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Just ask her. If the gift is supposed to be shared since there is only one?
And you wanted to check and make sure so that you could explain to them.
Next: Does she do this every year? Gifting only ONE of your children? If so, then... by now, the kids must know too. That this Aunty only gives ONE of the kids, gifts.
You can't just be saying every year.... "oh it must be a mistake..." and make excuses for this Aunt. Because, if it is happening REPEATEDLY, then it is on purpose.
And then, you need to explain to your Eldest child, that for some reason, the Aunty is only gifting the girl.
That some people don't have nice manners.
Or, you simply ASK that Aunty.... "Why" she only gifts one child, the girl... and not the other child, who is older????
Simply, ask her.
Why guess or make excuses for her.
Just ask her and get to the bottom of it. Especially if this happens every year.
Or the other gift got held up in the mail.