Does you sister happen to put her phone in the back pocket and then sit on it? This sounds like soldering spots that got loose. Bake the mainboard in the oven. Seriously. But only the mainboard, not the screen and by god not the battery. Disassemble everything, clean the USB port from dust (you don’t want the dust to melt and burn) and otherwise follow this guide: overclockers.com/the-oven-trick-repairing-your-br…