If you read my last phrase, you'll see I said; "the first one", so yeah no, I listed one contrary of what you say.
But if you need more;
-Sister was found dead and calcinated in the arms of her mother.
-Shawna don't have features that looks like the sister.
-She even didn't have a single burn scar on her.
-She's not bothered at all about being on a plane when even if she suffered from amnesia in the theory that she weirdly survived, a trauma like that affect the brain in many ways even if she wouldn't remember why.
-They're more chances that Molly is the sister that any character in that game.
-If she's the sister, in that case it would be fucking weird that she's a character you encounter and you can give her a fake number, then if you gave her a fake one, you one sees her again. If that was the plot twist all along, kinda dumb to have one thing that literally shaped the story (the plane crash and MC being closer to Nana, Aunt Amy & Aunt Liz), being something that not only can be missed, but also being on a throwaway character.
-It was already said at first that Shawna was supposed to be a one-off character because Moonbox was trying to have a sex scene in the first update to test things, and was supposed to leave it like that but since the character was really popular, she would be seen one last time. If the theory that she is the sister is true, it would be kind of weird, awful and dumb to have one more scene with her and then, never saw her again, right?
-Also, did I said the sister was smoking hot (I mean, literally)?