How do I build my own life without leaving her behind?

If breaking up came easy, there wouldn't be a song telling me that it's hard. I am also pretty sure Adele wouldn't have won so many Grammy awards. For many, breaking up calls to mind a half-empty closet, a bed that's far too big, or not hearing a familiar voice when you walk in the door. For me, it conjures a fear of wandering around grocery stores with a basket filled with different cereals because they all look good without my sister there to tell me that caramel flavored Cheerios are a bad idea. Either way, it sounds really lonely, and I'd rather not. Once upon a time, she and I were just like every other set of sisters: We fought for our parent's attention, played Barbie dolls, and had friends apart from one another. She is three years younger, five inches shorter, and in every way my other half -- the louder, middle-child half. Yet when we reached high school, my parents decided to drop out for a bit. They gave us a car to share, equipped us with matching ATM cards, and "fend for yourself" night became a daily occasion. I can't remember if we minded: We picked up the pieces, learned to shuttle ourselves to and from soccer practice, made our little brother grilled cheese sandwiches and just went about things until one or both parents reappeared. When I went away to college and she to boarding school, we took a break (you know, to meet other people). Yet, in the past few years we've fallen back into it. Hard. Aside from the occasional fight when I borrow her clothes without asking, things are actually going pretty well. We've moved in together. We've taken on some relationship-cementing hobbies. I've become almost dependent on her constant presence and she on mine -- even though she has a serious -- 5-years worth of serious -- boyfriend. I think from time to time he resents that I am the one that she leans on the hardest. Alternatively, I think he is appreciative that I am there to take her home and hold her hair back when the night takes a turn for the drunk. I allow him to live far away and be there, without truly being there. For my part, I've grown disgustingly accustomed to her maternal instincts. Like the best sort of mom, she reminds me to make my bed, put away my laundry and calls me obsessively when I break curfew. The other day she removed a glass bottle from the garbage can and reprimanded me because This Family Recycles. I am slowly realizing, however, that this level of intimacy is threatening just about everything outside our bubble built for two. If the thin casing was formed out of survival, now it has matured into something rigid and impenetrable. Our existence is one without an authentic need for anyone -- except each other. Needless to say, it makes having friends, boyfriends, an independent life complicated. Recently, it has become like the anti-freeze in the Gatorade bottle of her relationship: odorless and slowly poisonous. Her other, other half feels no need to be there for her, and worse, has no idea how. Her dream of a white picket fence involves being able to look out the window and stare into my kitchen. I don't think he shares the same Hitchcock fantasy. Somewhere I know this is wrong, but I know I would readily accept her invitations for sleepovers -- even if it did mean that her other, other half was forced to the couch. So I'm hardly innocent in this co-dependence. I use it to distract myself from the need to grow up and become a person who does her own laundry and has relationships that move beyond text messages. It's easy to blame her. Or shove the responsibility on her little shoulders. But willing myself to do something I don't really want to do feels impossible when she is there to sort my clothes and consume my emotional reserves with her needs. The thing about family (real or anyone who feels that way) -- the addictive thing ­-- is that they allow you to be the truest version of yourself. They knew you when you wore braces and sat next to you in the plastic teacups at Disney World. They are there for keeps without pretense or a filter. Yet this closeness can also be antithetical to change. I recently applied to a job across the country -- on a whim, or a hope that maybe I would take a risk and go. I had a phone interview while walking across the Brooklyn Bridge with my sister in stride and felt like I had to say no. What else could I say? She was looking at me, silently challenging me to say anything but. Yet, as I watch her relationship dissolve into silly fights and my own Playskool romances continue along as little else but gifts of gummy bears and sex, I wish we could dissolve the casing or expand it -- at least a little. It would be hard. She makes it easy to remain unattached. I never have to be alone or decide what cereal I actually want -- and to be honest, caramel Cheerios sound sort of delicious. I don't know whether being miles apart would give us the right sort of breathing room to stand on our own. I want us to be ready, or I want to be.