YTA!

I don’t think u are an asshole for being against cheaters or wanting to stick up for ur friend! That’s actually pretty amazing!

I do however think u are the asshole for making Julia’s comfort a priority over others their comfort! During the wedding people will attend that love your brother and his two children (and maybe even his new partner)

And they might feel very uncomfortable knowing that one of their grandchildren/nephews etc isn’t going to be there because off julia!

I understand u inviting julia over your brothers new partner, I mean julia is your best friend! Even though I also understand if ur brother isn’t going to come if his partner isn’t invited!

You shouldn’t just only be a best friend to julia but also an aunt to both your nephews! Don’t exclude one!

Anyways congrats with ur wedding!