Передавай привет (своей) [сестре]! - if you say ты/thou to the other person.
Передавайте привет (вашей) [сестре]! - if you say вы/formal you to them.
Are you familiar with the ты/вы difference? In short, ты is informal and used between friends and family members, but now increasingly in other situations. Вы is used if more formal contexts e.g. between colleages and people who don't know each other very well.