I have two sisters, one who is two years younger than me and another who is three years younger than her. 

From the time we were two and four, people were mistaking the older two of us for twins. 

I've always been small for my age, and my sisters have always been large for their ages. If the youngest were just a year or two less younger than us, we'd probably have been mistaken for triplets a time or two. 

It gets annoying, after a while, when you're fairly regularly called your sister's twin. Your sister, who copies you. Your sister, who is obnoxiously loud and hurts your eardrums, who is too aggressive and shoves you around and sometimes hits you if she gets angry, even after it is discovered that she gets hyper and therefore even worse when she has dairy, and rarely allowed access to it. Your sister, who always runs into the bathroom as soon as you start towards it. Your sister, who always tries to hang out with you at school, and makes you feel like shouting at her "enough is enough, I'll try to tolerate you at home but not during recess," except you're just too shy and quiet to do that especially when she's talking a mile a minute about things you don't even want to hear and you can't get a word in. Your sister, who is always arguing with your other younger sister, and often winds up dragging you into the argument, either dues to something she says or because you just can't stand them arguing anymore but the only way there's even a chance they'll listen to you is to shout at them that they have to stop arguing, which of course never helps.

Luckily, as we've grown she has figured out that certain things she used to do made me hate her, and slowly started doing them way less often, to the point where she is tolerable and sometimes even possibly someone I could consider a friend. At first it was just random half-hours every three months or so, but eventually she realized that she was doing something right in those moments, and started doing it more often. Plus we have not been seriously mistaken for twins for a couple years now. It probably has something to do with the fact that we both used to have blonde hair (not yellow-blonde, leaning towards a dirty blonde) but around when I was in fourth grade my hair started becoming darker and settled at a nice, deep shade of brown, while her hair started becoming darker just a couple months after mine, and quickly settled at a shade of blonde-brown that is clearly different than my hair. Also, she has grown enough that she is actually taller than me. The top of my head comes up about to her eye level. 

The third sister, the one who is 5 years younger than me, currently comes up to about the same spot on me as I come up to on the sister I have been telling you about.  Her hair darkened to a brown similar to mine. In two or three years, it is quite possible that people will start mistaking us for twins, despite the fact that I will be nearing the end of high school and she will be just starting middle school. I just don't grow enough to outpace my younger sisters. Currently, to me, the middle child is "my younger sister" because if she is bigger than me I can't call her little, and the youngest is "my little sister" because she isn't taller than me yet. But I wouldn't be surprised if she passed me in a couple of years. If not in a couple of years, she will be taller than me (or exactly the same height as me) by the time she stops growing. If she isn't, everything I know about how this world works will need to be reassessed.

Someday, I think, I will get or make us a set of matching shirts. Mine will say "little sister" and theirs will say "younger sister" and "youngest sister." And then we'll watch people get confused every time we wear them.