Diya might be antaras bestie who for some reason or due to antatas insistence or as antaras doc might b staying with the roys. Mj and dia might have had a fling or cud be mjs mistress also without antaras knowledge.
Mjs mom must b knowing so she is cold towards him and dia.
Antara doesent know about them for sure as she said to dia yest" tum meri dost ho ki mj ki".