Parents of a grown up woman are not responsible for their actions. But they were not blameless. They actively encouraged her behaviour (to an extent Aru did that too).
And they forced hand of their other daughter to sacrifice her life to save their neck.
And for that they actually were let off the hook easily by just blaming Mishri.
It seems everyone including Aru forgot that it was NOT MISHRI who came up with this idea. And Aru's Mom did not suggest this. Initially she did not ask Aru if she is willing to do it. She assumed that Aru will do it because she is her "acchhi beti" and directly ask Mishri (without asking Aru), a 20 odd year girl who was in a bad place then.