You should read through the Twitch Developer Agreement https://www.twitch.tv/p/legal/developer-agreement/ and ensure your app would meet Twitch’s legal requirements.
For things like watching live streams in an app the most important thing is that the ONLY way to do that is by using the Twitch Embed https://dev.twitch.tv/docs/embed as any alterations to the Twitch Player, or trying to circumvent the Twitch player, would violate that developer agreement.