I’d say Ruby’s closest sibling is Smalltalk personally.

People say this mainly because of the brace-enclosed blocks of code being functions. However, that is really just a syntactic sugar for lambda (and can even have explicit arguments: (lambda (x y) ...) <--> {|x,y| ... }.

there's the "everything is objects and messages sent to them" bit too, which is definitely from smalltalk. ruby took the good bits from several languages.