Answer: rivalry

AnswerCrossword Clue
RIVALRY

It's not unusual among siblings

RIVALRY

Army versus Navy, e.g.

RIVALRY

It's common among siblings

RIVALRY
Competition.
RIVALRYCompetition
RIVALRY
Yankees vs. Red Sox, e.g.
RIVALRYSiblings may engage in it
RIVALRYKeen competition between teams