I wouldn't call it hostility though the fact can become a reason for jokes by other children because they can stand out. There is an expression "Что я, рыжий, что ли?" which literally means "Am I a redhead or something?" but should be understood as "Am I not like everyone else?".
Sorry to say that but волос can also be plural. For example: "You have a lot of hair on your head" (meaning the amount/quantity) would be "у тебя много волóс на голове". However bear in mind that вóлос still means one string and if you say "у тебя много вóлос на голове" everyone will still understand what you mean but it will just sound strange... hope it helps
Yeah. сестры́ is the genitive singular, and that's what being used here. The pronunciation, on the other hand, is сёстры, which is the nominative plural.
According to Forvo, the pronunciation is very close to the word for red meaning "rouge." рыжие pronunciation = rouge-yeah. https://forvo.com/search/%D1%80%D1%8B%D0%B6%D0%B8%D0%B5/ru/