Why is my ipad glitching and touching the screen itself?

so, a couple of months ago, my ipad mini got pretty badly scratched and cracked, but it still worked fine. until about three days ago, the iPad just started tapping random areas, and going into apps. It started with just little moments, like it would spasm and then be fixed, but now its just non stop. its also only on one half of the screen, which is odd. its 4 years old, but my sister has the same model, the same age, and her's' still works fine. PLEASE help, i have tried turning it on and off. and i cant reset it, because i have to log out of find my iPad to do so, and i can't even unlock it.