Hey!! Welcome to the forum.
1. We don't know yet why Maya hates MJ. We see constant flashbacks of 10 years ago of a man (with no face shown) and Maya being intimate, later that someone torturing her and possibly even killing her baby. In addition to that, that person was also connected in the deaths of her father and brother, Manas. Also, it has been heavily assumed that Roys are connected to these horrible events in Maya's lives as everyone in Roy mansion, barring Rishi and Rudra, had known Maya 10 years ago.
2. It has not divulged in the show what is her plan and why is she using both of MJ's sons.
3. We don't really know why Rudra hates his father. But, we have seen flashbacks of Rudra's gf cheating on him 10 years ago and that incident is related to his father.
4. MJ does not know Maya exists in their lives as of now. He suspects that someone is using his sons as there have been a lot of turmoil in their lives lately and Rishi is acting unlike himself.
5. The younger girl in the house, I believe you mean Diya, who is constantly with Antara, Rudra and Rishi's mom. Apparently, Diya is a college friend of Antara, who lives in MJ's house for some odd reason. We are all sure there is a back story as whenever Antara and MJ are close, she is jealous.
Honestly, you haven't missed too much. They have spent the past month setting up the story by getting Maya in to Roy's lives. We have not really been shown what happened in the past of any of the characters.