AITA for inviting my husband's sister and niece behind his back to our wedding?

My (26M) husband (27M) has always been very close with his sister and his niece. She's a single mom, and his niece looks up to him as a father figure and quite literally hero-worships him. She's the sweetest, most polite teenage girl you can ever imagine.

So, cue my shock when he says that she isn't invited to our wedding reception.

I asked him why. Our wedding reception doesn't have any sort of drinking or alcohols in it, and it is pretty kid-friendly in general. What we have done is made it childfree for children under the age of 5, who are usually the noisiest of the bunch, and even then, for parents who have no other options, we've allowed it.

My husband told me that some of his best friends have toddlers, who they have to leave at home, and they are disgruntled. They've told him that if he wants to make it childfree, he should make it completely childfree; that no one under the age of 18 should be allowed. And thus, he caved under pressure.

I told him to re-consider, and how hurt his niece might be. He refused to listen.

So, I called up my SIL (she was in tears) and told her that she has my full permission to bring her daughter to our wedding.

On the day of the wedding, my husband saw his niece and was overjoyed to see her. It was clear he didn't actually want to disinvite her, it was due to peer pressure. However, his friends were enraged. A few hours later, he told me that his friends are criticizing him for making double standards, and that it was my fault for inviting his niece.