Well, I believe the first step is for you to contact several of the people involved and let them know of your copyright registration.
But before we get into that, I will provide you with a little background information on copyright law relevant to your situation.
(1) You live in New York, USA
(2) Your sister has created/authored musical works and registered copyrights in same and she owns all the copyrights (may not be accurate - see below).
(3) At least one person or entity in Poland is using your sister's musical works without her permission.
What can be done?
SELECTED COPYRIGHT LAW
The reproduction/selling/transferring of copyright material without the copyright owner’s permission is copyright infringement unless such use/actions qualify as fair use.
I see little change of what you describe as being “fair use.”
Therefore, what you describe would be copyright infringement under US law as such actions violate provisions of both the Copyright Act and the Digital Millennium Copyright Act (DMCA).
What about Poland? What happens if your work is copied in a foreign country? Does the law of the foreign country apply or does US law apply?
There is no “international copyright” that automatically protects an author’s writings throughout the world (but all is NOT lost).
That said, generally speaking, if there is copyright protection in other countries such depends on the national laws of that country. The copyright laws of each country are concerned only with actions that take place within that country. For example, US law will apply to copying within the US no matter where the work was originally written or published. Similarly, Polish copyright law applies to anything published or performed in Poland regardless of where it was originally created.
Thus, copying in the United States will be regulated by US copyright law copying in Poland will be governed by Polish Copyright law.
Therefore, while there is no international copyright, most countries offer protection to foreign works through international copyright treaties and conventions. The two big ones are (a) the Berne Convention and (b) the Universal Copyright Convention (UCC).
The US and Poland are a signatories to the Berne Convention and the Berne Convention treaty requires all signatories to recognize the copyright of works of authors from other signatory countries.
Poland is a contracting party of the Berne Convention treaty (January 28, 1920). Therefore, Poland is bound to honor your American Copyright claims in your sister's musical works UNDER the copyright laws of Poland.
POLISH COPYRIGHT LAW http://www1.american.edu/initeb/rw9257a/property.htm
Copyrights in Poland are protected by the Law on Copyrights and Rights Related of February 4, 1994. The new law meets contemporary international standards and corresponds to the principles of free trade in intellectual property and is consistent with the protections of the European Union Copyright Directive.
The 1994 law was substantially revised in June 2000 and the scope of copyright protection has been considerably broadened. The new law covers not only the protection of traditionally understood author's rights, but also related rights.
The law provides for new rights and new owners of these rights who are able to decide how the outcome of their work is to be used and are able to derive financial benefits from this outcome. The new owners include produces of sound and video recordings, TV, and radio stations as well as artist-performers. The new law provides protection of intellectual property in the area of science, technology and manufacturing, including computer programs, industrial designs, etc.
YOU ARE NOT ALONE
Piracy of CDs (both via wholesale and resale venues) remains the largest source of piracy in the recording industry. The level of piracy in the recording industry reached 30% in 2001, with approximately 90% of CDs smuggled into the country coming through its eastern border. In a two-month period in 2001, over 100,000 pirated CDs were seized by authorities, and throughout all of 2001, Police enforcement authorities recovered approximately 1,001,180 CD units, amounting to more than $37 million (US) in losses.
OK, your US copyright rights are protected by Polish copyright law - assuming Poland enforces such laws. Now what?
We have just found out...accidentally....that her music from her first album is being used in Poland for several television series and films including a soundtrack release of a popular TV series in Poland being sold on Amazon and Ebay with her song on it.
I would contact the legal departments of these TV Stations and Film producers and inform them of your copyright claim and ask to see the documents giving them written permission to use your musical works (there should be none).
I would do the same for this "Polish music producer". Your next action will depend on their response.
I could not locate a US copyright registration under "Kelly Saint Patrick" but I did find one under "Kelly Skrzysowski"
Information provided below
Type of Work: Music Registration Number : PAu003618756
Registration Date: 2012-05-19
Application Title: Pussycat V.1.
Title: Pussycat V.1.
Description: Electronic file (eService)
Copyright Claimant: Studio to Be Named, Inc.,
Transfer: By written agreement. Address:XXXXX, Apt. 7i, Rego Park, NY, 11374.
Date of Creation: 2009
Authorship on Application:
(1) Elektrica, pseud. of Vera Bobrova (author of pseudonymous work); Citizenship: United States. Authorship: music, lyrics.
(2) Salme Dahlstrom; Domicile: United States. Authorship: music, lyrics.
(3) Kelly Skrzysowski; Citizenship: United States. Authorship: music, lyrics. Rights and Permissions: Cassandra Spangler, Esq.,XXXXX Suite 704, New York, NY, 10001 Names: Bobrova, Vera Elektrica, pseud. Dahlstrom, Salme Skrzysowski, Kelly Studio to Be Named, Inc.
Note the following from the above information:
(a) copyright claimant is Studio to Be Named, Inc.
(b) Copyrights transferred: By written agreement.