I'm holding a grudge against my sister because of her daughter
Can you explain the issue between you and your sister?
I think my sister is not kind enough to her daughter.
Holding a grudge against your sister in this case is not going to help her daughter. If there is serious abuse happening, then you need to get the daughter out of the house and into a safe place. But if you just think that your sister is being too strict, even then holding a grudge won't do anything.
Wtf I should leave her daughter to her? That's so narrow minded.
What I mean is that you holding a grudge against your sister will not cause anything to improve. Depending on the seriousness of the situation, you either need to talk to your sister about how you feel, or in an extreme situation, try to save her daughter from her.