I think this will eventually be possible by using a formula field, when the option to refer to self in a formula becomes possible (it was requested here at some point but I can’t find it at the moment).
So for example, a type with a relationship to itself (parent-children) can have a lookup field (Siblings) which is:
Siblings: Parent -> Children
which as you say, returns all children including itself.
But then use a formula that filters out itself:
Siblings excl : Siblings.filter(Name != this.Name)
@mdubakov @antoniokov
Is self-referring on the horizon?