You seam to be focusing your anger at Verizon and not your sister. Verizon followed the rules and required IDs and passwords to access the wireless account. You and/or your mother are supposed to safeguard that information.  Your sister has committed the felony by stealing your mothers identity for personal gain.

You say "I am very dissatisfied with how VZW is handling an illegal and fraudulent change on my account that has shafted me out of 500 dollars.'  My question to you is who made the illegal and fraudulent change to your account?  Verizon or your sister? Keeping in mind your sister had all the correct IDs and passwords,

I'm sure if you went to Verizon and wanted to make some changes to your account, and you had the correct IDs and passwords, you would be upset and angry if they put you through the 3rd degree because they thought you were committing fraud.

Good luck to you, (seriously) whatever happens,