>>881317
Personally, the reason I like sisters in the first place as far as romantic interest goes is exactly because it's like a childhood friend taken to the extreme.
To me the appeal of a childhood friend is the way she's a part of your life, something that's been a fixture so long it's hard to remember or imagine her not being there. Growing up together, and growing close as friends, until eventually you realize your feelings for each other are romantic as well. The love confession isn't the start of a new relationship, but something built atop the old one; you were friends, but now you're both friends and lovers.
A sister is similar, but on another level. She's even more a natural part of your life, and you have an indelible bond with each other that can never go away.
The problem with non-blood-related sisters is not that they're glorified childhood friends, but that they're a watering-down of sisters. They exist not for their own merits, but because the author chose not to make them blood-related. Not because a childhood friend wasn't enough, but because a blood sister would, the author believes, have been too much. Calling them glorified childhood friends misses the fact that they could legitimately be a tier above childhood friends, but the unavoidable comparison to real sisters pulls the foundation out from under them.
Long-lost siblings are also bad, since although they do have the benefit of the inherent connection to each other, they don't have the gradual build-up. That build-up is the key that distinguishes sibling and childhood friend romance from other, more sudden archetypes.