If adopted siblings marry each other is it incest?

I’m unclear exactly what you’re asking. If the adopted siblings are biologically related, then absolutely it’s incest. If two unrelated children are adopted by the same set of parents and raised as siblings, then I think it’s up to their culture whether or not it’s deemed acceptable for them to get married or not, but I think the vast majority of cultures today would still consider it incestuous.

If adopted then no not incest. Same as for step siblings.

If they're no biologically related in anyway, no. It'd just be creepy as hell.