Is it considered a sin not to travel to see one's mother in another country, knowing that she moved there specifically to enable her son to visit her, and given the apprehension about traveling between countries?

There is no sin upon you for not traveling to your mother if you fear harm, based on the Prophet's (peace be upon him) saying: "There shall be no harm nor no harming." Apologize to her gently, and explain your eagerness to meet her. Maintain ties with her through what is possible, such as calling her, checking on her well-being, and sending gifts.