The fantasy says no, edge for him till he begs you to suck that hard on you’ve given him.
But fantasies aren’t always meant to come true. This guy is like a brother, and losing that, which you risk if you go further, is a real possibility. So I think you have two options.
First, just keep it as it is. Enjoy the sexual tension, don’t burst the bubble, keep teasing him, and being teased by him. Sometimes that’s beautiful. As a denial slut you know the bittersweet agony of wanting something you don’t get. He’s now on that list.
Alternatively, yes, cum, clear your head, and then have a grown-up talk about it. Make sure he’s clear-headed too (no, you can’t help). Say you love having him to confide in, ask him how the way you act makes him feel, what it makes him want. Tell him sometimes it makes you want to do more, and talk about that too, as adults. He might be totally cool with that, and if you are too, then maybe it’s okay. And in the cold light of non-horny day he may well not be and that’s good too and you know to rein it in a bit.
If he’s truly into denial too there’s an interesting reciprocal offer you could suggest, that might even make things less likely to go where they shouldn’t.
‘Hey Cuz, ever thought about wearing a cock cage?’
Imagine that, you edging, him caged.
Something tells me you’d make a very good keyholder.