The reason you can't see your sisters iPhone in the Find My application is likely because you have different Apple IDs and aren't sharing location with each other via family or Find My People (formerly Find My Friends incase you are on old iOS).
In this case you should go to https://icloud.com/find in a browser and log in with the Apple ID associated with the device you are trying to locate, not your own.
I hope this answers any questions you have, but if not you can feel free to reply and I will try and get back to you. If this reply helped you please mark that it solved your problem, thanks in advance!