Honestly one of the interesting things to me about the siren episode is it truly doesn't have to be about sex to be as hard hitting.
Let's say the siren wasn't about sex then, doubtful, but let's go with it. It still is about revealing Dean's deepest desires for a relationship. At Dean's deepest core, his desire is for Sam. Full stop. You can paint that platonic if you'd like, but to state it's about anything other than Sam you'd have to blatantly be ignoring the text itself.
Don't get me wrong, I think the the title of the episode itself "Sex and Violence" truly says almost all there is to say about the implications upon Dean and Sam's relationship, but the thing that a lot of people forget about horror is sometimes, it's genuinely not about sex. People can cross boundary lines that can be just as damaging without actually raising a hand to you. Dean doesn't need to be attracted to his brother, for the horror of the fact that Dean wants his brother more than any other type of relationship or connection to come through.
But like what does that really mean though? It means that Dean's need and desire for Sam goes beyond the simple physical. Dean doesn't want Sam's body, he's not after a fuck, he isn't after "some bitch in a g-string", including Sam in a g-string. Dean wants Sam fully, everything that makes him up.
The horror of it is about the fact that Dean doesn't just want to possess Sam's body, Dean wants to possess and keep Sam himself.
Like think of how much deeper that goes.
But on the flip side, as an asexual person, I hate to break it to people but sometimes, sex is just sex. It's not a be-all-end-all definer of relationships or how they function. Dean wants Sam. Dean wants Sam. Who cares what form it's in, healthy siblings don't want their other siblings. They care about them, they enjoy their company, they have them as an integral part of their life, but they don't statistically want their siblings in the way Dean and Sam have always seemed to want each other.
Sam and Dean again and again show that to them their life itself revolves around the other. They have shown again and again that they will distance themselves from most other people or even reject others around them if it puts them losing the other at risk. Sam and Dean have shown again and again an unhealthy need to possess the other in some way, to control the other in some way, to literally keep the other in some way.
Siblings don't do this. Like, full stop. Unless like Sam and Dean they cross the lines of emotional incest so often they might as well just have a picture of themselves in the dictionary definition.