A lot of sibling headcannons feel more like truthing than an rpf/rp headcannon. Like you can enjoy two people's banter, and feel free to make a character headcannon (or an rpf headcannon!) post on whatever platform you use. However, I don't think we should be writing things like 'these two are like siblings' in people's youtube comments. We already know we shouldn't write 'I ship them' in people's comment sections, so why does that not extend to other relationship headcannons? It will be read as truthing, more than as a fun little headcannon, and I just don't think we should be pushing creators into a found-family-like dynamic, when it's common sense we wouldn't do the same with romantic relationships.
I could talk more about the way certain posts are discussing the sibling headcannons in ways that sound like truthing to me, but tbh as long as it's out of the creator's view, it doesn't really matter. The important point I was trying to make, is that we should not be writing about how two creators act like siblings in their own fucking comment sections. Thank you.