I think as Rashami and Asim are what we call power pair whereas Arhaan is a liability on Rashami atleast in the house the way he is playing.
So sid sees that as a competition to Sidnaaz and moreover I have seen him commenting on Rashami even in Sana ki shaadi task when she was fake wife to Bhau..that time also he couldn't control himself and showed his jealousy/irritation??