I am now playing as Molly and I really do not comprehend the idea of her flying around as a bird, then becoming a big fish and now a monster. In each case eating animals and then people. Does this concept come together in the story? Perhaps I am not seeing the big picture here.
You've hardly started. Each sibling has a story. None of the other siblings "go flying around" but there is a theme that links all the stories.
If you don't care about spoilers and would rather understand what happened in the game before playing through the game yourself, this video
-- "The Villain of Edith Finch" --
explains much of what happens in the gamehttps://www.youtube.com/watch?v=6bMn4CoyUkM
What does the title mean? Which Edith does the title refer to? The woman who for most of the game is the player character... or the grandmother who is referred to as "Edie" in the game?
The answer to the question of "What Remains of Edith Finch" depends on which Edith you think the title refers to. If you think it's the player character, then the answer is the same as in the aforementioned video. If you think it's the grandmother Edie, then the answer is something else.