Because sharing is caring and web is an awesome place, Jardson Almeida published a massive UI kit containing 50 iOS app screens. It includes diverse sign in/sign up app screens available in Sketch format. Here is some additional info:

Operating system: iOS
Format: Sketch
Screen size: iPhone 6 (750 x 1334)
Font family: San Francisco Display
License: Creative Commons 4.0

You can use this great resource to speed up your learning process and get inspiration for some new design works. Show the author some love by spreading the word.