A classic case of Identity theft
The majority of identity theft cases are made by family members, closed friend or neighbors – people we know. Your case is no different.
What your sister did is very wrong, and unless you confront her you stand to loose. Not only will your credit get ruined. By this time the account is probably in collection, and appears on your credit report. This is why you can’t get any kind of credit, including a cellular contract.
The collection agency is likely to file a law suit against you, and will probably get a judgment.
The only way you can avoid all this is to file a police report, and with this police report you can have remove the negative information from your credit report, and the collection agency off your back.
This involves reporting your sister to the police. It’s not an easy thing to do, but considering what she’s done to you, I think that you are left with no other choice.
See ../how-to-report-identity-theft for a checklist for clearing the damage of identity theft.
P.S. Do pull your credit report and check it thoroughly. Your sister may have opened other accounts as well. The sooner you deal with it, the better.