Here's why:
* Eldest is used to refer to the oldest person in a group of siblings.
* Elder is used when comparing two people.
Since Ravi is being compared to his brother, we need to use eldest.
The sentence could also be written as: Ravi is the elder of two brothers. This is grammatically correct, but it is less common to use "elder" when referring to the oldest in a group of more than two siblings.