Is the paternal uncle considered a mahram for a woman who travels and stays in his country with her female colleagues?

Your sister's travel to a country without a mahram (definition) is an abominable act, and she must repent from it. The mahram is a condition for travel only, not for residence. If her stay is with her female colleagues in a safe place, then there is no harm upon her. Sheikh Ibn Baz states that there is no harm or objection to a woman residing in a country without a mahram, especially if her work is among women and protected from men.