"न त्वं देहो न ते देहो ज्ञाता ज्ञेयं च नात्मनि। अज्ञानादेतदित्याहुः ज्ञानेनैव परिक्षयः॥" — Bhagavad Gita (Chapter 13, Verse ...
Not only are they a built in best friend for their siblings, they're automatically another (slightly cooler) parental figure ...