While it can be debated who made bigger mistake Dharmi or Mishri. It is notable that while Mukhi is not treating his sister's mistake lightly, Aru is not seeing her sister as she is.
What Mishri did was inexcusable (her apology yesterday was a height of immaturity, she compared small accidents to a life altering decision on his behalf and also on behalf of a stranger (Aru)).
But at least what Mishri did was a reaction to something dreadful happening to herself and her brother. In spite of that Mukhi acknowledged that the mistake was a huge one and is not trying to justify it.
Aru on the other hand is trying to justify Dharmi's behaviour.
Even if it is granted that Dharmi ran away due to someone's instigation, it also meant she did something she wanted to do.
I guess somewhere being a Panchayat and solving matter of this kind makes Mukhi see the situation in more clearly.