No specific diagnostics for FaceTime, have you and your mom do this with the devices though.
1) Open Settings
2) General
3) Reset
4) Reset Network Settings
5) Enter password and wait to restart
6) Try Facetime again after you rejoin the WiFi Connections
Beyond that what you can do is download Skype and attempt to see if the issue occurs there, or Tango Video Chat. If the issue is present between both devices on different video chat programs it's one of your connections or possibly the devices in which case you need to go to the local Apple Store for whomever is closer and the Genius Bar can test it out for you if your phone is less than a year old or you have AppleCare coverage. If it works just fine using Tango or Skype then it's the software and it will hopefully be patched quickly to fix this.
Side note, when did/have you upgrade(d) to iOS 7