Missmarie, I'm with you on this one. I believe showers and gift registries are for first babies.
I think your plans for a casual, get-acquainted-with-baby bbq (no gifts required) is a better idea in this situation.
Still, this shower is already planned and is going to take place.
I wouldn't want to embarrass my brother by objecting to the arrangements, or by not showing up. It is what it is, and as you've said, there is a lot to celebrate and be excited about. I would attend with a gift, and that would be my baby gift (rather than taking one to the hospital or saving it for later).
I think this is where I fall. I've mentioned recently that a friend plans to throw her cousin and his wife a baby shower--it's his first child, but her fourth, with her older children being teenagers. It really bothers me to listen to my friend's reasons for wanting to throw the shower--like that the parents "can't afford" to buy baby things all on their own. I have no idea if that's true or not, but I think it's an insulting thing to say about someone--I don't think it speaks well of them. Also, I don't like the idea that I'm somehow obligated to support someone else's baby. And my friend openly admits that both parents are socially anxious and that while they might use "dad's first" as an excuse for the shower, in reality Dad would rather skip it.
But I think ultimately her intentions are good, wanting to celebrate the baby and the fact that her cousin is having his first child. In her mind it seems to be a way to include the wife and her older kids in the family, rather than excluding them. Personally I think they could have gone the "meet the baby" route (presents not obligated) rather than the "shower" route (presents obligated) but honestly I'm just an observer on that event.
Since the deed seems to be done in the OP's case, I wouldn't bother objecting, as it will probably just cause hard feelings. I think I would also try to attend, more for the sake of family harmony and thinking of it more as a general celebration of family. I would probably take a gift, maybe something smaller than I normally would, and not get them anything else later (like at the baby's birth or whenever you were planning to). Not with a punitive or judgmental attitude, just thinking, "Okay, this is what they want and how they want it, so I will support that, and not feel like I have to do anything extra."
My friend also said, "Everyone was going to get them a gift anyway. So they can just bring the gifts to the shower." Someone in another thread talked about the "push vs. pull" of gifts--being invited to a "shower" means being asked
for a gift, and I don't necessarily like that, even if I was planning to get them a gift on my own. Some people don't feel that distinction is important, though, and that's fine. It might be because I personally find gifts very powerful (love languages and all that) so being outright asked for them seems a bit bold to me, and should only be done carefully, under certain circumstances.