Your Sister Didn’t Cut You Off. You Both Stopped Trying.
There was no final argument between you and your sister, nobody sent the long message explaining why the relationship was over, and neither of you ever said you were finished. You started talking less, and then the time between conversations got long enough that whoever called first would have been making a statement by calling, so neither of you did.
Maybe you used to talk every couple of months, nothing deep, just enough to know whether her car was still running and whether she'd gotten the promotion. Then you noticed you were the one dialing almost every time, so you stopped for a while to see whether she'd reach out, and she didn't reach out, and you didn't call again. Half a year went by before either of you would have had to admit anything had changed.
Some estrangements end with somebody saying so, a message or a phone call you can repeat later when people ask. Others end without anybody deciding, no falling out, no last straw, contact stops because both of you quit doing the small things that had been keeping it going, the birthday text, the photo of her kid you'd forward on a slow afternoon.
The second kind is hard to explain to anyone, because people want somebody to be responsible for it. They ask who stopped speaking first, or what she did, or what you did, and you can't give them an answer they find satisfying, since the honest one is a few hundred small decisions spread across six years, made by both of you, most of them not really decisions at all.
Adult sisters have to build the contact themselves, and it's more work than anybody expects, because when you were younger your parents were doing most of it without either of you noticing. You were both at their house every few weekends, you heard about her job from your mother, you showed up to the same birthday parties whether you'd have chosen it or not. Once those weekends stopped, the two of you had to decide whether you wanted enough of a relationship to make the calls yourselves, and you might have found out that neither of you was especially good at it.
A few missed calls became a month, and a month became six months, and when one of you finally sent a happy birthday, the exchange stayed short because too much had happened in between to catch up on comfortably. Neither of you was going to open with the divorce, or the eight months you were out of work, so you both kept it light and let it end after four texts.
Old disappointment is usually part of it too. Maybe you stopped reaching out because you were tired of being the one who cared more, tired of driving four hours to her place while she'd never once made the drive to yours, and she might have had her own complaints about you, the anniversary you forgot, the four hundred dollars you didn't send when her transmission went, the time she asked you to come and you said you'd see. Each of you waited for the other one to prove she wanted the relationship, and neither of you did.
Sisters can stay in a standstill like this for years, because there's no event to resolve. You aren't angry enough to confront her, and you aren't close enough to miss her most days, and you think about calling every so often and then decide it would be strange to call now after so long, and she might be making the same decision on the same afternoon without either of you knowing it.
You can still miss what the two of you had, back when she'd call from the car on the way home from work and stay on the line until she pulled in. You can also see that the two of you had less and less to say to each other for years before the contact stopped, that the last few conversations were already mostly logistics about your parents.
There's a pull toward assigning all of it to whoever called less, missed the last birthday, or left the last message unanswered. Sometimes the details hold up, and one sister did keep showing up while the other one turned her down every time, and pretending it was mutual would just be a lie that lets somebody off. In plenty of these, though, the distance really was both of you, both less available and less interested in repairing something that had gotten uncomfortable.
You stopped trying too, and admitting it gives you an accurate account of how the two of you got here. Maybe you were tired, maybe you were done being the one responsible for keeping it alive, maybe what you were putting in had started to feel like more than what was coming back.
If you decide to call her, you don't have to pretend the six years didn't happen. A relationship that ended this slowly isn't going to come back in one phone call, and you'd both have to figure out whether there's enough interest left to build something that fits who you are now.
You might also decide the distance is fine. Some sisters stay out of touch because both of them have built lives that don't include each other much anymore, and nothing blew up, so neither one is obligated to manufacture a reconciliation because other people think sisters are supposed to be close.
There was no terrible fight. The relationship got a little less from both of you every year until there was almost nothing left to keep, and her number is still in your phone, and you'd have to ask your mother what grade her daughter is in.