Kumud is a typical mom(adoptive mother who has been forced by her husband to mother a adopted child). She has one favourite son and that is Michael.
Arjun was not adopted with her consent. Rudra just brought him one fine day and said this is our son. It was not her decision to adopt. She has not completely accepted him as her son, she just see's him as Rudra's son beacuse of which she is his mom.
She is also angry and jealous that Rudra always praises him and likes him and not Michael because he is good at work and takes responsibility for business. She wants Michael to inherit all business and property and is insecure that Rudra may make Arjun incharge of everything as he likes him and feels he is more capable than Michael.
Bur Rudra in reality does not really love Arjun unconditionally. He loves him because he is a capable person in business. And kumud knows that and will try to make Arjun look incapable of handling Rudra's business and throw him out of business and house.
Actually i think what she is doing is good fr Arjun and Rudra in long term
Why?
For Arjun his greatest realtionship and weakness in life is Rudra(not Rashi). His happiness, sadness all emotions depend on whether RPS is happy or sad. Hence if RPS only kicks him out of house and tells him never to come back he can go out in this world and develop a independent, crime free life
For Rudra also it will be good because once Arjun leaves the house RPS wll understand how hallow his family is Kumud, Michael, Jignesh will not even care about his happiness and sadness. Hence he will realize Arjun's importance in his life as a son/family and will treat him with love.
The losers in this fight will be Rashi and Romit who for no fault of theirs will be seperated from Arjun even though they genuinely love him as a brother/family member. So there will be no one to protect, take care or even speak to them in that house once Arjun leaves.
Arjun Always Gave Them Love 
Even I hate also Kumud Aunty 
