Tell mom the truth meaning mom would tell Arthit, then they would pity Paeng, and started treating her like she'd…
It's not like she wasn't pitied. Everyone made fun of the fact that she didn't have parents or wasn't brought up well (except Arthit & his friends) even without knowing the real truth of her parents' death. I'd rather Mum pity her than to treat her badly. Also if Dad explained to Mum properly and requested her not to reveal, I believe that Mum would have tried not to do that.
Tell mom the truth meaning mom would tell Arthit, then they would pity Paeng, and started treating her like she'd…
I get Dad's logic too. But when he saw how she was being treated, he should have told Mum. At least that would have made Mum more empathetic & sensitive to Paeng's needs. Like she needed love at that time and Mum's behaviour at that time worsened things.