1. I feel like there's a bit too many coincidences and convenient details in her story. The Madelyn name, both girls being writers, father reappearing just to give a name, mother being dead, the girls having different mothers…
2. I think that getting an identical replacement for both the wife and the daughter is a bit too "on the nose" and belittles Madelyn herself as a character, like Ben said. Her whole character right now is Ella's sister, Maddy's namesake, not a separate person. A twist where she's an impostor who grows to love the MC and regret her ruse could shift the attention from the dead and back to her.
3. I like twists in general. Except those making a LI a different kind of girl entirely, I hate those.