OP you really should tell your brother.
You have nothing to feel embarrassed about - all you're doing by letting that understandable but off-base feeling take hold is cutting off your major sources of support at the same time as giving him the chance to get his own lying version of events out there.
If your brother is his friend, then the main thing he's going to feel, after anger and worry for you, is betrayed himself - your H has shat all over his family, his sister, and their friendship. So especially if your H plans on parading his 'new relationship', you need to let your bro know what's going on. How is he going to feel when one of their mutual friends asks him when you split, and what's the deal with the giggly fake on your H's arm? Pretty stupid, if he knows nothing about it.
Don't let your bro be caught short with this and especially don't water down the story because he's your H's 'friend' - all that will happen is he'll feel a twat when the gory details come out, and if you've minimised it and he's stayed neutral, he'll feel shat on too.
Tell your bro everything - EVERYTHING, just how awful he's been over the split as well as the gory details. Your bro would prefer, I'm sure, the judge him on the truth - then he can decide whether he stays friends, and can also arm himself with a few choice phrases in reply to mutual mates asking what the hell's going on.
This may sound odd, but you should be pleased that he's acting such a twat. Firstly, his behaviour now - parading this immediately, lying so pointlessly to his parents - isn't going to win him any favours. Oh, he may get support on the surface, but everyone from his mum down is privately going to move him down and you up in their estimation with every twatty, insensitive, inappropriate move.
Secondly, it's really becoming clearer with every event that this guy really, really is a complete bellend. I'd have more belief in this being potentially the start of a real, deep relationship if he weren't acting like this - if they were being sober, slow, currently in hiding basically, knowing what they'd done and being horrified by it... that's how adults would be reacting now. And adults form adult relationships. Not these two. How utterly immature would you have to be to do this, break up a family and start a work affair, and parade it in town the day after? They appear as teenagers. And what do teenagers do? Act utterly immaturely and OTT, declare undying love, and then five minutes later it's all over. Just like this. So, I say again, beware that in a month, this guy could be crying on the doorstep. Or, if not, don't be surprised to hear in 6 months that Fakey Bubblebabe has moved on to someone else.
I've read a LOT of threads on here, and I don't think it's a coincidence that, when you hear a story like this of an absolute humdinger of a loser husband, with a perfectly lovely wife, kids etc that he's screwing over, sooo often you hear her say 'I can't believe it, we've been together since teenagers. Yes. Indeed. You took up with a loser, but because you were so young, with little other experience, you didn't actually realise that until now.
I have a feeling OP that when the dust is settled, you're going to look back on this moment and give thanks. You are a quality person and a good parent. He's shown himself to be neither (same goes for her, hair or no hair!) You will move on from this faster than you think, I suspect. And he will move where he was always going - to the bottom.