Technically she could but she'd need physical access to your unlocked device for a bit to install the software. If she has installed actual spy software I doubt turning off location would help.
OTOH small battery powered GPS trackers that can be put in your vehicle are pretty cheap these days, there are ones you plug into a computer whenever you can get to the device and download where it (you) have been. They also have ones that you put a prepaid sim card in and it sends the owner updates every 5 minutes or whatever they set it at depending on how long they want the battery to last. A bit more complicated but still cheap as hell are ones that are wired into your car. Very tiny and easy to do with those "add a fuse" things, they even tell you the exact time the vehicle is started and stopped. On virtually all of them with sim cards you can set up "zones" where if you leave or enter a certain area it will automatically send an alert to the owner by text or email. So if she suspects you are doing her sister or an old girlfriend she can set have it alert her if you go near them without having to check your route every day.
I, uh, just asked a friend and he gave me all of this info.