AITA for not letting my SIL’s kid order an adult meal for my wedding?

I’m getting married in 2026. The meals are going to be steak, chicken or vegetarian with Mac and cheese and chicken tenders for the kids.

My fiancé’s brother and wife RVSP’d with three adult meals and one kids meal. I know for a fact they have two kids, a 12 year old and a 7 year old.

I thought they made a mistake, so I called my SIL to straighten things out. The invitation said that anyone under 12 was automatically going to get a kids meal.

The reason why we have it is because catering for this wedding is pretty expensive. It’s $150 per plate for the adults.

My SIL wrote back and said that her 12yo is turning 13 a few weeks later and it doesn’t make a difference.

I wrote back and said it was because it’s an extra $150 on our bill.

My SIL then pointed out that I didn’t make the 12yo a flower girl or a bridesmaid even though her sister is, so the least I could do to make it up is let her have the adults meal.

I told her that if she wants her daughter to have the chicken dish, then she can switch plates at the reception.

My BIL and his wife didn’t like my reply and my fiance is telling me to let him handle his brother.

Still, AITA or are they being entitled?