Siblings, one invited to cousins birthday

[Karenm25]

I've two children 14 months apart. My sisters sons birthday falls in between them. They all go to the same school and play together. My sister invited my son but not my daughter to her sons birthday. My sister said it was "boys only". My daughter would be hurt and I'm annoyed on her behalf. Am I being unreasonable?

@JanetheObscure

I'm not sure I understand why boys/girls only parties are so awful, if that's what the child wants. My DCs requested single sex parties for a few years at primary school, but were soon back to wanting mixed as they got older.

Isn't it perfectly normal?

I think it’s quite Normal for a kid to list the names of kids they want to come to their party and for the all to be the same sex. However to say you’re automatically excluded from a place on the list just because you’re the wrong gender is wrong on so many levels