The little girl in India was actually attached to her parasitic twin that died in her mother's womb, but whose body remains were sourcing nutrition from the complete twin' body. The parasitic twin was partially absorbed by the healthy twin's body, until birth.
The dolphin's rear fins are NOT of any parasitic twin.
How does one tell if they are a mutation, an aberration or a vestige? And what creature looked like a dolphin but had legs and feet... that evolved into a dolphin? Before one can state with confidence that the fins are vestiges, one must be prepared to state what animal evolved into a dolphin.