Assuming 'Ista story' is an Instagram story, it's not legal to watch without permission. Instagram is designed to give users control over who can see their stories. If you try to use some unethical or illegal methods to view stories, like using third - party apps that claim to let you view private stories, you could be breaking the law and also violating Instagram's terms of service.
It depends on what you mean by 'Ista story'. If it's Instagram - related, generally it's not legal to watch someone's story without permission. Instagram has strict privacy policies. If a user has set their story to be visible only to certain people, and you access it through improper means, it's not okay. This is to protect users' rights to share content privately.