Indem er die schwiegermutter fickte, bewies der schwiegersohn, dass er nicht unpotent ist

The mother-in-law suspected her son-in-law that he was impotent and his penis could not become erect. That's why the mother-in-law called the son-in-law home to check, but the son-in-law turned out to be so clever that the mother-in-law turned the mother-in-law into a bitch and fucked her.