Need some character clarification

Actually, if I'm not mistaken, I believe that Deven Verma and Farida Jalal were close friend's of Pooja's parents. They were not related to them. So after Pooja's parents' death, they raised her as their own. So to answer your question, no, Pooja and Ajay are not cousins.

"If at first you don't succeed, then skydiving is not for you!"

No, they are not cousins. In Indian culture, most elders are referred to as "Uncle" or "Aunty" out of respect. Akshay Kumar's parent's raised Pooja after her parents (their close friends) died in a car accident. This storyline is actually kind of similar to Preity Zinta in Dil Chahta Hai.