While Payal has been shown more hostile towards Aarti, Palak seems a lot more open. In the mall she was more irritated that Ansh was "lying" than being possessive of her father. And when Aarti started to explain she was willing to listen and accept the explanation given to her. So I feel like we should all take a minute and separate Payal and Palak in our heads, because it seems there are two of them for a reason and will have two distinct sets of character traits.
Payal's different behaviour could be because she is older as well and may vaguely remember Arpita better than Palak does, or she may feel "responsible" and so protective and motherly towards her sister, which is why she is not welcoming Aarti's currently invasive presence. It might not be that the girls are rude or badly brought up, just that they are reacting badly to Aarti in their lives when nobody has taken the time to really explain to them.
Remember that we have not seen their normal behaviour, only their reactions to this marriage. On the other hand, we know and understand Ansh a lot better, in and outside the context of the marriage and he is positive towards it. In reality all the kids have issues with their parenting.