Disclaimer: I haven't watched any previous episodes before Shivi wedding so my views are based only in the episodes since then
So,ever since the twin weddings happened, Rishita is becoming the center of attraction for all the wrong reasons but I don't blame her at all and find her behaviour quite understandable.
If we look at things from her POV, she has been brought up in a nuclear family, a very protective and pampered upbringing(I might be wrong in the pampered dept), she has never experienced a joint family or how it's different from a nuclear setup, she has never experienced the dynamics between multiple family members all living under one roof.
Now she falls in love with a guy. Her own family are dead against thier marriage and even the guy's family is getting him married somewhere else. After many trials, she escapes and stops the wedding and get married to the man of her dreams. Now being brought up in a nuclear family and being newly married obviously she wants to spend some alone time with her husband which naturally every girl wants to do but instead they are thrown into a family outing with over zealous family members and no privacy at all. For someone with no family experience, so many people can be suffocating.
Now coming to her behaviour with Dhara, I feel she thinks Dhara was getting Dev married so naturally she will resent her a lot. Also she doesn't understand what Dhara's stature is in the household and in Dev's life. For her, she is another DIL albeit an elder one so why give her the stature of a mother or follow her orders.
Now let's take up Raavi, Rishita rightly said all she wanted was to marry Dev, she never asked Shiva to marry Raavi nor she is the reason for their misery, so why should she feel guilty for them when they married on their accord and she has every right to enjoy her hard earned marriage.
Again I'll say I'm not defending her behaviour but just trying to understand where she is coming from. She is rude, self-centered, disrespectful, childish and thoughtless but it's because all this new dynamics is so foreign to her that she can't understand how to navigate it. Also let's keep in mind that both Dhara and Raavi have known the Pandyas since childhood but Rishita has just met them
But the one person I hold responsible for her behaviour and all the mess is Dev. We know how big a role he played in the wedding fiasco so let's not discuss that but even before the wedding, he never explained Rishita about his family or his bhabhi-ma. He never explained how drastically different his upbringing and family is from Rishita's and what all she has to encounter. I feel Rishita just needs a little time to adjust and learn abt her new family. The family is expecting too much from her too soon.