The Fenris Twins are not mutants, but were bio-genetically altered to gain special powers. The twins first assumed the Fenris identity in Uncanny X-Men (1st series) #200. In Thunderbolts: International Incident, Andreas revealed he was born in Madripoor, not Germany -- this would certainly apply to Andrea as well.
Andrea was murdered by Helmut Zemo in Citizen V and the V-Battalion #3. Her body was recovered and her brother flayed her skin and used it as a tanned leather handle for his blade as the Swordsman, allowing him continued access to their shared powers. A clone of Andrea appeared in Thunderbolts (1st series) #122-124, the result of Arnim Zola's cloning technology. She was killed by the Thunderbolts, however, who believed her to be one of the Skrull impersonators during Secret Invasion. She later resurfaced in The Illuminati #2, presumably as another cloned body.