In your case, you should send them a "not me" letter as detailed below.  You are right, you shouldn't be getting calls from these crooks.  Your sister should send them a debt validation letter as detailed below:

You need to send them a certified, return receipt debt validation (by law they have to go back to the original creditor to prove you owe the debt), cease and desist calling, or "not me" letter (think of it as the best $6.59 you ever invested).  Also send it first class mail in case no one will sign for it.  Make a notation of that fact at the bottom of the letter.  You also need to report them to the proper agencies to which I have provided links to.  Then you need to go out and get a consumer lawyer to sue them on a contingency basis (no money out of your pocket) and let them pay you for the harassment.  If everyone did these three things, these crooks would be bankrupt and we would all be the richer for it.  Use these web sites to find an attorney in your state that will probably take the case on a contingency basis (no money out of your pocket up front):  http://www.consumeradvocates.org/
http://www.consumeradvocates.org/find-an-atto ... tates_value=All

Everything else you need to know follows including at least two agencies where you should report their criminal activities.

REGIONAL ADJUSTMENT BUREAU
Note they have been sanctioned by the FTC for using deceptive and illegal practices to collect debts, but it looks like they are ignoring it:  https://www.ftc.gov/enforcement/cases-proceed ... ment-bureau-inc
Posted In:  http://800notes.com/Phone.aspx/1-866-536-4622
Web site lists them in Texas:  http://www.rabtx.com/contact-information.php
BBB gives them an accredited A+ even though they have 32 complaints, 1 negative review, and an action by the FTC stating that they used deceptive and illegal practices to collect debts (BBB removed 45 complaints and the notation that there was an FTC action against them):  http://www.bbb.org/memphis/business-reviews/c ... cordova-tn-2196
They are actually located in Tennessee.  The Tennessee Dept. of State has 5 listings for them 4 of which were revoked:
Name:    THE REGIONAL ADJUSTMENT BUREAU, INCORPORATED

Status:    Active    Initial Filing Date:    07/30/1971
Formed in:    TENNESSEE    Delayed Effective Date:
Fiscal Year Close:    June    AR Due Date:    10/01/2016
Term of Duration:    Perpetual    Inactive Date:
Principal Office:    1900 CHARLES BRYAN RD STE 110
CORDOVA, TN 38016-5167 USA
Mailing Address:    1900 CHARLES BRYAN RD STE 110
CORDOVA, TN 38016-5167 USA
AR Exempt:    No    Obligated Member Entity:    No
Shares of Stock:    1,000,002

Registered Agent

Agent Name:    SMITH JR, JAMES S
Agent Address:    1900 CHARLES BRYAN RD
STE 110
CORDOVA, TN 38016-5167 USA

Other complaints:
http://www.consumeraffairs.com/debt/regional_adjustment_bureau.html
http://www.complaintsboard.com/complaints/regional-adjustment-bureau-c158605.html
http://www.ripoffreport.com/r/regional-adjustment-bureau/memphis-tennessee-38016/regional-adjustment-bureau-this-company-may-or-may-not-be-what-it-seems-memphis-tenness-669068
http://ficoforums.myfico.com/t5/Rebuilding-Yo ... FD/td-p/3668537
Looks like they get sued a lot:
http://law.justia.com/cases/federal/district- ... 1460/460118/35/
http://caselaw.findlaw.com/us-11th-circuit/1451859.html
https://getoutofdebt.org/92321/the-regional-a ... mplaint-1297303
http://www.ca5.uscourts.gov/opinions/unpub/15/15-10655.0.pdf
http://www.law360.com/cases/4e31ab9c904343545b000001
And some more government actions against these criminals:
https://www.finance.idaho.gov/ConsumerFinance ... ease_Desist.pdf

It is incumbent upon them under the law to prove that the debt exists and that you owe it, and (this is the important part) that they have the legal right to collect it.  You are not obligated under the law to prove that you don’t owe or that it is paid.  

Federal law (FDCPA) requires them to send you a letter (US MAIL ONLY) postmarked within 5 days of their first contact that contains their name, physical address, the creditor’s name, and the amount of the alleged debt.  It also must contains “mini-Miranda” telling you that it is an attempt to collect a debt and that all information will be used for those purposes.  The one other important thing that this letter must also have in it is that you have a right to dispute the debt within 30 days of receipt of the letter and if you do so, all collection activity must be stopped until the debt is verified.

First, you should make a complaint at this Federal Agency, and while there you should also read up on how debt collection is supposed to work as well as what your rights in this matter are:  http://www.consumerfinance.gov/

Also file a complaint with your State Attorney General's office.
List of State AG’s offices:   http://800notes.com/faq/attorney-general


Also file a complaint in Tennessee here:  http://www.tn.gov/commerce/topic/commerce-file-a-complaint