O rings are great. They're also easy to forget when doing gear checks. Sounds like the O ring slipped and occluded the 1st stage regulator. Or the O ring was fine, and the first stage reg failed. You know, I assume, that your depth did not cause the O ring to slip?
Events like that can be scary, but the ascent shouldn't have been that difficult. Why were you both sharing air? If you had just descended, his brother should have had abundant air for both to ascend safely.
The one and only potentially dangerous equipment failure I've ever personally witnessed, ever, was an O ring failure. We were about 60-ish feet down, at the end of a dive off Cozumel, when the guides O ring totally blew out. That will empty your tank in short order, especially at the end of the dive. I was (as usual) at the back of the group. The other 5 divers just looked (which reenforced my long held opinion that recreational divers, in general, don't practice "oh crap" stuff near enough) while I swam past them and handed my primary regulator to the guide. This is a tech/rec training difference. Rec divers are taught to hand over the secondary reg. Tech divers hand over the primary, generally with the purge valve pressed during the transfer; we keep it flowing so they can breathe instantly, and do not turn loose till it's in their mouth. Because I just took a breath off it, and there's gas coming out as it's transferred, the person
KNOWS, 100%, that they will get something breathable. And our primary is on a 10' hose, which means once she was on it, and I was sure she was ok, I could signal ascent to the rest of the group, shoot my SMB (which you cannot safely do while hugging each other the way you must to share air on rec gear) and head for the surface. And since I always dive in tech gear, I had two 100CF tanks instead of 1. On a recreational dive, I will generally have 2400-2500 PSI left after the first dive, so there was no need to even skip the safety stop.
I was kidding, and I hope you are too. If you're not cave trained, please do not ever go into a cave. There is a reason caves that are regularly used by divers have signs like these:
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I have never passed out drunk, I have only been (mildly) drunk once in my life, and no responsible dive shop should allow people to dive if they have been drinking. So I am safe from this prank, at least.