My advice is to ask your brother.

We put formal on our wedding invites, because DH's side is extremely casual and have been known to turn up to wedding in thongs and shorts. But we explained to my side that suits and ties would do, because they most certainly would have turned up in tux and top hats (I wouldn't have minded that, but it would have been impossible to get DH's side to go to that extent).

generally speaking though, it is completely acceptable nowdays to wear a suit with either tie or bowtie (or some variation), dress that hangs lower then the knee, button up shirt and slacks for boys and party dress for girls, to a black tie event.