You Were Sisters. You Were Never Close.
When people hear you don't speak to your sister, they usually ask what happened, and they're expecting an argument or something she did that you couldn't forgive, so the question arrives with an answer already built into it. You struggle to give them one because you can't remember a time when the two of you had much to say to each other, and there's no falling out to describe.
You grew up in the same family, you know the same relatives, and you can remember plenty about being children together, the house, the car your father kept fixing, an aunt who said something at a wedding in 1994 that people still bring up. Your sister remembers some of it better than you do, including arguments you were too young to understand at the time, and she'll mention them as though you were both there for the same evening. All that history between you, and almost none of it involved either of you saying anything about what was actually going on.
People expect sisters to call each other, share their problems, and want to spend time together after they've left home and nobody's making them. Someone says "But she's your sister," as though you've forgotten, and you're left explaining a relationship to a person who already decided what it should look like before you said anything.
You might have been far enough apart in age that you spent your childhoods doing entirely different things, and by the time you were old enough to have a real conversation with her she had a job, a boyfriend, and an apartment across town. Family visits gave you updates about each other, secondhand and about a month late, and neither of you developed any interest in filling in what happened between them.
If you helped raise your sister, you spent more time checking her homework than talking to her about anything you liked, and you knew when she needed lunch money because you were the one expected to find it. Some weeks that meant two dollars out of what you'd made babysitting on Saturday, handed over on Sunday night so she wouldn't be the child at the counter with nothing, and nobody asked where it came from or offered to pay it back. Years later she still calls when she needs help, and she'd have trouble telling anyone what you do for work.
Sometimes there's less to explain, you had separate friends and spent almost no time together unless somebody made you, and the two of you could get along fine at dinner and then go back to your own bedrooms without either of you wanting the conversation to continue. Living in the same house made contact automatic, you saw each other every day for eighteen years and never became friends.
You might still feel affection for her and want things to go well even though you almost never speak, and hearing she was ill would worry you, and you'd be glad to know she got a job she'd wanted. Most of what you know about her life comes secondhand from your mother, who assumes the two of you have already spoken and tells you anyway.
The distance becomes obvious once you're adults and seeing each other takes somebody making a plan and picking up the phone, and for years your mother is most of the reason you're ever in the same place, inviting you both over on a Sunday and telling each of you what the other one said. You catch up over dinner, you go home, and neither of you expects to talk again before the next time she arranges it.
Eventually your mother stops doing the calling, because she's seventy-eight and can't be on her feet for a whole afternoon anymore, or one of you moves far enough away that a visit takes a flight and two nights in a hotel. Contact thins out because neither of you has the habit of calling for no reason, months go by, and by the time you notice how long it's been you wouldn't know what to call about.
You can lose contact this way without either of you deciding anything, so when someone asks what she did you find yourself reaching for an old disagreement you can barely remember, because a fight is easier for them to follow than years of nothing in particular. An incident sounds more reasonable to people than admitting you never really had a relationship with her.
You might also notice how little your daily life changes once you stop speaking, and if contact was already rare there isn't much to adjust, she was never the person you called when the car died or when you needed somebody to sit in a waiting room with you. You still talk to the same people about the same problems, you just don't get news about her.
Admitting you don't miss her much can feel uncomfortable when people expect you to be wrecked about losing a sister. You might miss her occasionally, or feel sad remembering something you liked doing together when you were nine, and you can leave it there instead of working the feeling up into something bigger so other people will accept it.
There's still sadness about the relationship you thought you'd eventually have, especially if you assumed adulthood would take care of it, that once you both had jobs and rent and complicated years the two of you would find each other. Hearing another woman describe an hour on the phone with her sister can remind you how much you'd like somebody who knows your family without needing the background explained. Your sister knows the family, and you spent about fifteen years explaining yourself to her anyway.
Some sisters do get closer as adults, usually because one of them keeps calling until it stops being awkward, and that might still happen with the two of you or it might not. If you and your sister were never close, you can say so when someone asks, without producing a betrayal to justify it, because the years before you stopped speaking already looked like this, months between calls and nothing much to say when they happened.