Instigation or encouragement or even direct help can be only useful if the person to whom it is directed at is willing to accept all of it.
Dharmi WAS marrying Mukhi for his wealth and to spite her ex-husband.
She did not think even once about her parents or sister when she ran away.
She had no qualms about doing a theft.
While Rami knew that Mukhi will not call police, Dharmi has no such knowledge. She was ready to leave with that money and leave her family to be taken in custody by police.
And she had no regret about doing theft even now. Her only reason for regret is that she should have talked to her parents.
So when Aru says "Meri didi aisa nahi kar sakati", she is being blind by sibling love and not seeing Dharmi for what she is.