1. Going to school is a huge problem

Not necessarily intelligence-wise, even if your older sister is a complete idiot or your younger sister would prefer to work as a street artist in Brooklyn instead of go to college, but just think about this: after your parents are done paying $50k a year for four years for one sister, they immediately have to do it again for another four years with no gaps for recovery. You hear that? That’s the sound of their retirement plans going out the window (and don’t think for one second that they’ll let you forget it).

2. Special events take a lot of bargaining

While we’re on the subject of school, getting your family to attend your respective milestones can be a bit of a trade-off. Inevitably, you’re going to have certain things happen at the same time, and you’ll have to pick and choose what’s more important for your family to go to. Oh, you’re graduating from high school? That’s cute, my college graduation is the same day. What’s that? You’re graduating AND you’re valedictorian? …Nevermind.

3. You can work as a team to tackle (most) obstacles thrown your way
  
Whether your little sister has that same, godawful math teacher you had when you were in high school, or your older sister needs to find a way to convince your parents to let her spend Spring Break in Cabo, your sister will always be your partner in crime. But don’t think that means you won’t totally owe her when she’s telling you all the ways to get an A in that sucky math class, or when she helps you perfect your innocent pout and subsequently lets you borrow that cute bikini to go to Cabo. When tragedy strikes, such as the death of a loved one, you two can always count on having each other to lean.

4. You love each other no matter what the age difference is

Until she uses up the last your $17 hair product and doesn’t tell you (or worse, blatantly denies it), or you accidentally forget to charge her laptop after you borrowed it and she has a presentation to give in 15 minutes and no charger. Then it’s war. Full-on, no holds barred, sleep with one eye open, take no prisoners, war.