Ruhi's grudges are definitely fine; she should defintely have grudges towards her parents. But hating her parents for that isn't something good; and in this case her hate is more towards Ishita(concluded based on the facts she was aware of). What I always believe is "You won't lessen your pain by hurting others".
And though being a child Ruhi suffered the most, but didn't IshRa suffered too? They were away for 7 years, and Ishita was believed to be dead same as Ruhi. Wasn't it itself painful for parents to perform death ceremony of their child every year? She is away from parents being alive, isn't that a punishment for parents.
On top of all this, even if Ruhi wants revenge what will she get in return; she has no plans of revealing her identity; so she is not gonna be a part of family even if its ShaRam and IshNi. She would be back with Nidhi, and Nidhi's treatment towards her would never change even if she does what Nidhi hoped since long.
Being angry on someone is different, but hating someone is entirely different. There is a big difference between the two.
P.S. No offence, but i really believe that hurting the one who hurt you doesn't lessen your pain; it will always remind you the ugly past. Life is to move on, not to get stuck with something. If you stick probing your past you can't move ahead.
