Watching movies with my sister

I think it's fair to say my sister and I have a complicated relationship. She's 4 years younger and has a selection of learning and behavioural problems which make her difficult to relate to and very hard to grow up with.
Occasionally she surprises me, I don't think she's stupid, far from it, sometimes she's much smarter than people give her credit for, and she certainly knows how to twist things to her advantage.
We communicate via old Buffy re-runs. It sounds bizarre, but since she struggles to converse with people and used to have to be prompted to speak on the phone (still not her best communication platform), it's the best we can do at times.
She has the world's shortest fuse, and a tendency to go completely nuts for no real reason at all, and because, unlike me, she struggles to find the right words, tends to lash out physically first. She also does karate, so when she punches, it really hurts.
On Saturday she came home from a week camping in the rain with the Guides (she's a Young Leader), feeling pretty miserable. While she was away my dad was supposed to decorate her room, it hasn't really got very far, so she's been sleeping downstairs on the couch.
Last night, I offered to untangle her hair, and plait it, so it didn't continue to look like a sparrow was living in there. I put on the animated-but-not-really-suitable-for-kids film, 9. I'd bought it on a whim (something I must stop doing) and got to work on the knots, turning it into a smooth, shiny plait. She has gorgeous hair.
Then we watched the rest of the film sat on my bed. Something we never do. We only talked about the film, that's how it works. We communicate about what's happening on the screen in front of us, because she struggles with small-talk, has a limited short term memory so asking her about things is pointless, and doesn't really have a lot to offer. I lent her a book. She was reading it last night.
Today she went out, she's back now and in a grouchy mood, it's weird; the moments where we get along are so few and far between, I should be used to that, but I already miss the girl who sat on my bed in her pjs and dressing gown last night, telling me how much she liked the film and thanking me for brushing her hair.