OK, the only way that you can interfere with your gran's file from your PC (and for her to see you on her PC) is for you to be both on a LAN.
On your PC, go into Windows Explore. At the bottom of the list in left hand pane click Network. Do you see your gran's PC listed?
Do the same on your gran's PC, does she see your PC?
If the answer is no, then she is being hacked by an outside source. If she is, it may be too late, but do a full virus scan on her PC (and I mean full, with all options ticked). I take it she has antivirus software installed?
Do another full scan with ESET online scanner (Google them)
Do another full scan with MalwareBytes (Google them & install), you will have 2 free weeks to use their Pro version).
If you can see each other on your network. On your gran's PC, in Windows Explore, right click on her main drive where her data is stored (probably C: drive). Select Properties, Click Sharing Tab. Does it say "Shared"? Go into Advanced Sharing. Uncheck "Share this folder".
OK to everything.
I'd reboot the PC.
Bet you cannot see her now (if if it says you can, bet you can't access it)
Hope that helps