Basically im just worried that something else is wrong with my PC and that's what's causing my SATA controller to suddenly crash like that and maybe killed the GPU. Any input is appreciated.
It looks like you did all you can do to narrow the problem down to your GTX 970. Both computers fail with the GTX 970 installed. Your computer works fine with her graphics card using your PSU. So the odds are good nothing else is wrong, and your GTX 970 is toast.
At this point I would
not keep experimenting. If the GTX 970 has developed a short, it could cause excessive current through the PCIe slot and motherboard or the PSU or both causing further damage which would not be good - especially if you fried your sister's system too.
Any chance the GTX 970 is still under warranty?