Ganga did not told the truth to Mannu at right time due to her fears that he might get hurt and might misunderstand her . So when the truth came out in most disastrous way , Mannu is behaving exactly in the way G has feared then why it is unnatural ?
Mannu is extremely hurt now , the world he thought was his own has collapsed suddenly in front of his eyes within a span of few hours. He was already little unsettled after knowing J's first marriage . J had already lost the reverent status in his eyes after that disclosure. The new sudden development about his parentage has completely shattered him and he is behaving absurdly. He is now drawing his own conclusions which is not abnormal considering at teen age kids have a rapid imagination.
With time he will come to the terms of with new realities , then perhaps he will realize his mistakes and ask forgiveness from his family. His realization can gradual or sudden both as per wish of writers.
Btw this is a fictional show , to get maximum impact they will dramatize every situation as much as possible whether realistic or not.
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Btw it looks bad that Mannu was misbehaving with the same DS who opened an advanced cancer treatment section in her hospital to save his life.