To do this with DS and expect no contradiction?

[MidwifeyForLifey]

SIL's DD is having a princess party. When I asked what the boys were coming as, she said superhero's.

DS (a toddler), would look very cute in a prince outfit so I've gone along and purchased an ivory coloured Christening type outfit, complete with little tights and ballet shoes And of course, a little crown. I bought it at the time I was told about the party and nothing was really said apart form an eye roll.

She really doesn't approve and has gone ahead any purchased him a little superhero outfit, despite knowing I've already bought the prince outfit.

AIBU to think this really isn't fair?

She also 'doesn't agree' with boys doing ballet dancing etc, but that's another story for another day.

Judging by your responses you are going to do what you want anyway so as well as attention seeking at the party you have come on here doing it too.
He is a 18 months old for goodness sake. Just dress him in some comfy play clothes for a party. Your outfit sounds very uncomfortable and I am sure no one will be interested in what he wears.

I'm with the OP.
She heard the party theme.
The other guests will be dressed in sparkles.
She bought a great outfit for a boy, with sparkles.
The party mum doesn't think boys can wear sparkles.
That's the party mum's problem not the OP's.