The presence of fins in both species represents an adaptation to the same environment (i.e., sea), but in no way does this adaptation means close evolutionary homology between fishes and mammal cetaceans (whales)
Homologous structures are parts of the skeleton of organisms are similar in structure. This gives the impression that their is a common ancestor. Mostly the structures are not similar and the function may or may not be the same in appearance per-say such as fish fins and whale flippers. The important part of homologous structures is that they have a skeletal similarity.
This, and the emergence of other structures is the product of evolution so each organism can have the best chance of adaptation to their environment. This ensures the survival of a species and the continuation of the next generation.
