After reading Carl Sagan's The Demon-Haunted World, I came up with a theory for why my childhood memories are so thin while my sister's are rich and cinematic.
I think it's because I'm a poor storyteller and I live in the future. I remember facts, not narratives, and I rarely sit around reminiscing. Without a story to hang them on, the facts have nothing to stick to, so most of them just fade. My sister is the opposite — she turns her past into intricate, entertaining stories and replays them constantly, which keeps the details alive.
The interesting part is what that does to accuracy. My sister almost certainly retains more true facts than I do, simply because she rehearses them. But the ratio of true to false memories might be about the same for both of us, for opposite reasons. My handful of memories are little burned-in facts with no story around them, so there's never been much opportunity to embellish. Hers ride along inside stories, and stories want to be good: a sharper detail, a cleaner ending, a tidier moral. Repeat an embellishment enough times and it quietly becomes the truth. Sagan's book is full of this — the satanic-ritual panics, the implanted abuse memories — and Michael Shermer makes the same case: we are pattern-seeking, story-telling animals, and the embellishments often do more work than the facts, by teaching a lesson or simply being worth retelling.
So I wouldn't say my sister has a better memory than I do. She's a better storyteller — which, when it comes to memory, might be nearly the same thing.