Wow! Just read your updates absolutely breathtakingly rude and inconsiderate behaviour from your brother! Let alone your nephew!
Definitely decline the invite now at most send a card.
As you say you now know exactly where you stand which is very low in your dns considerations.
Your brother has raised an appallingly rude, spoilt person he should be ashamed!
Where does all this leave your poor mum?
From the sounds of things they won't have even considered her transport let alone her comfort on the day.
If I'd EVER put any of my grans in such an awkward uncomfortable position my parents would have given me an almighty bollocking for being a selfish, rude and ungrateful little madam!!
My mums one of 6 my dad one of 5, all my aunts, uncles, spouses/partners and children were invited. Some couldn't make it for various reasons (mainly health related) but even though I was fairly sure before the invites went out that this would be the case I wouldn't have dreamt of not inviting them.
And the cousins aren't even all blood related! Several are my aunts/uncles step children but I was raised to treat people kindly and with proper consideration - which your Dn clearly has not been!
If there'd been previous fallings out/estrangement that's a different circumstance but that is quite obviously not the case.
Going forward cool civility. No need to be rude but certainly no need to ever put yourselves out for them again.
They've made what they think of you and your other brother and your families - including his own gran! Quite clear, treat him and his family accordingly.