Having had a sister myself; and two daughters of my own (1 1/2 years apart in age, I can tell you that it is VERY normal for sisters to have a "love-hate" relationship. While they love each other, they also vie for attention, friendships, and just about everything. Although they will continue to have this type of relationship, while at home, when they grow up and go their seperate ways (ie: one gets married, or goes of to college, or whatever), they will learn to appreciate each other more, and most likely will grow to rely on each other, for advice, friendship, etc.