I am suddenly taken back to seventh grade, diagramming sentences.
This sentence is structurally the same as "Julia takes ballet" with the addition of an adjective clause "his sister." I'm not an English major so I'm not sure "adjective clause" is the correct term, but is applies well enough.
Then it is easy enough to identify "Julia" as the subject of the sentence, leaving the predicate being the verb and object "takes ballet."
I normally don't give "the answer" here and typically leave some part unanswered, but there wasn't much to leave behind here!