Everyone behaved pretty badly in sone respects.
Your brother and SIL should have arranged a seperate meal to share their news. I suspect they didn't want the kids there as they knew only a few were invited and wanted to avoid questions, the same reason your db said could your husband watch the kids. To avoid revealing they weren't invited. Poor timing on their part as the meal was already arranged for family and poor to be annoyed you couldnt rejig your plans for theirs. Poor response on yours not congratulating.
Yanbu to be upset your dp wasnt invited however you should have let it go for your db sake. It sounds like your family all took umbridge there and got het up on uour behalfs.
How did they intervene? Im suspecting they bought up the money your dad gave. So I'm not surprised he got a check back and they eloped, though the cheque should have cone from them not inlaws. Does your brother know about it?
My parents tried to give money with guest conditions, i refused. It's very controlling.
They spent the money on a holiday (honeymoon), their wedding and relocating. The latter of which sounds most expensive. Not my choice or priority but it should have been a freely given gift. Not one with stipulations.
They eloped to save any more fuss. Fair enough.
Obviously your SIL is pissed off, while your db is burying his head and pretending nothing happened- not healthy but nothing to do with you to worry on that's their relationship.
I would talk to your db, yanbu to be upset but a lot of this sounds like all your faults and needs to be out in the open. I would also consider that your parents/other family may have said things to your brother that you aren't aware of. You seem to think your dad would never give money with stipulations yet you may be surprised. My siblings were when they heard.