This is a bit weird. I wss all ready to say you were being insecure, but the sending you for drinks thing is really horrible.
It's also strange that he's going quiet and apologising rather than being all "wtf, you think I fancy Sharon?"
When I was in my late teens, I knew two brothers, and brother 1 unwittingly got together with the girl brother 2 had been obsessed with for about two or three years. The whole situation was really odd, the brothers were in the same friendship group and school, so they spent all their time together, with this girl, and even ended up all going to the same local university. I'm nearly sure they flatshared for a while too.
You'd see them about, and brother 2 with the unrequited love would just be panting along after his big brother and this girl that absolutely everyone knew he was obsessed with.
They all seemed to get on OK though. I mean his feelings were obvious, the brother and girl definitely knew. It was weird, but something about the dynamic worked for them I suppose.
At a guess, in hindsight, I'd say Brother 1 was the leader and the one in charge, the girl liked having the two of them running after her, and brother 2 was one of those men who preferred having an unobtainable, ideal dream woman in his life, rather than having to get to grips with a real adult relationship
Last I heard the couple were still together after university, not sure what happened or how it all played out, but I could see it ending up in this sort of scenario, as my friend used to point out, who would want to be brother 2's girlfriend coming into that!