When God gives you a sister, He gives you a gift you may not have known you wanted, but definitely a gift you needed. I'm not saying that having a sister is easy. In fact, a lot of the time it's really REALLY REALLY difficult. Having a sister is hard because, most of the time, they are reflections of ourselves that we might not want to see. Chances are the things you find most annoying about your sister is very similar to something you do... that most likely annoys your sister. I may find it incredibly annoying when my sister freaks out about her homework, but I have to admit someone *me* set that example long ago. Then an hours later,Then I annoy her because I want to watch do something in the room while she does her homework
So you can see it's a vicious, never-ending cycle. But if it's this complicated to have a sister, why do I say it's a gift?
I will say, a lot of the time my sister is a gift given by your distant relative from the sale section of a store that has probably already been returned once and you most likely are already planning in your head how much pocket money you can make by returning it.
Even then, my sister can drive me to the point of being the gift that it isn't worth the price of gas to return, so it's really easier to shove in a box somewhere and re-gift to some poor unfortunate acquaintance whose birthday you'll forget in five years. This is a rather lengthy metaphor (you'll have to excuse the English major in me for using my extensive literary devices), but I'm guessing you get the picture. At times, most times, my sister and I don't get along perfectly.
Just because we don't get along perfectly, however, doesn't mean that every moment I get to spend with her is somehow a gift. Often, the gifts that are best for us in the long run aren't what we want in the moment. God sees things in a bigger plan that we ever could, and I believe that when He gives us a sister, He is giving us a friend for life (whether she wants to be or not). She is someone who can't and won't ever leave you. She might not agree with you, but more likely than not she'll back your play because you're family.
And that's what families do. They support you in everything you attempt, and try to help you out and pick you up if you fall along the way. Sisters get the extra burden/advantage of being that shoulder to cry on that isn't a parent, and is often someone who will best understand what you're going through in the toughest of times. So having a sister is a gift, even if it is one that half the time you want to donate to the nearest Goodwill.