how a person who has been calling his / her cousin with names like "Bhai" (brother) "Baji" "Behen" (sister) suddenly transforms his / her mind and heart for a love quite different in nature when his /her parents initiate to get him / her engaged with the same person of opposite gender whom previously he / she treated like his / her siblings.
Take this, i know a person who adopted a boy and a girl from different families. They grew up in the same house as brother and sister. Then they were married to each other and now they have kids.