hey I agree with u. one day uday said that dia & he are born on the same date remeber.this was in the starting when sammy tries to cooldown dia by giving rosses through everyone & the dialouge was " Aaj tumhara birthday hai kya? oops tumhara hota toh mera bhi hota na" So I guess they are twins. but why are they studying in different classsesš.
I think dia did not get admissions in the school ( admissions were full)š that is why she is studying in a lowerclass.