If maithli was a vampire (probably related to the raichands) - In that case abhay & maithli relationship must have been a taboo ,and either someone from vampire side or from human side must have killed maithli and maybe abhay too and at the last moment chand & family must have rescued abhay ,possibly by changing him into a vampire.
If Maithli was a human - In that case ,there must have been a resistance to their marriage or love . And maybe they were pursued ,and in that Maithli was killed in the forest while Abhay must have been in deathbed . At that time ,Chand must have spotted Abhay & probably changed him into a vampire .
However , i can't understand if raichands have been living from such a long time ,then they should have an extended family . How it is possible that only Abhay & his mom is there . And no siblings