They can only call as they are allowed by the FDCPA(Fair Debt Collection Practices Act) and if you think they are violating this, I would send them a cease & desist letter and file a complaint with the FTC(Federal Trade Commission). I have included a link to a site that oultines the entire FDCPA, and includes a link(in the blue section at the top) where you can file a complaint. http://www.ftc.gov/os/statutes/fdcpa/fdcpact.htm
If you send a letter to the agency calling, send it certfied, return receipt requested. Let them know the nature of his illness, and ask them to stop calling, as your sister is well aware of the debt.
Hope this helps, Kathleen Eads