The way I see Sam and Dean's relationship, even if we're going fully bare bones canon, no wincest layers or themes, is that it's very much a D/s type of dynamic. But like, a real one.
There's a matched give and take. Maybe Dean is seemingly more dominant and in control of what they do and how they do things, but that's because Sam has given it to him.
In a D/s relationship, the submissive actually holds all the cards and it's not one person forcing the other, its mutually agreed, I give up something- you take responsibility- we trust each other.
If Sam let's Dean push him around or be bossy its because he's okay with it, he's willingly handed over the reins and when things happen that he's uncomfortable with, or Dean doesn't do something that he (Sam) wants to do, he does put his foot down and make his feelings and desires known and Dean DOES concede to him.
Sam has power and influence over Dean, and he does use them, but he doesn't want to, or feel the need to all the time, because of the nature of their dynamic that he also enjoys and benefits from.
Now, is a D/s dynamic normal or even healthy for siblings to have, even if we're keeping it at a strictly nonsexual level? Idk, probably not. But that ship sailed for them long ago and the point is, that it is a mutually agreed upon, beneficial, enjoyed or otherwise desired way of living together. Sam isn't being crushed under Dean's controlling thumb (Dean and control is something for another day but I will be addressing it in time), and Dean isn't being manipulated by his evil little brother.
A D/s relationship may look strange to an outside observer depending on what tasks or stipulations are present to be seen in the first place, but a true D/s relationship, which I believe Sam and Dean to have, brings satisfaction, security and fulfillment to both parties, even though it may come off in unconventional ways.
And I'm not saying that they sat down and talked through all of this (as you should in real life), they might not even be aware of the extent of it. (The idea of them having terms negotiating talk, while possible, does not seem probable to me). But that's where they've found themselves, even if they accidentallied their way into it, and I think they're quite pleased and comfortable with it.
