I will assume that by you broke Katarina meaning you already made her falling in love with you instead of Mario. I don't know if you happen to skip the dialogues, but after Mario force Katarina to have sex with you, he said that he want to divorce Katarina and he also want your help in corrupting Emma.
Then next step you need to do is visit the Villa and straight to the Sauna to meet Emma. Do this 3 times ( once a week & no specific days ) and tell her that she's beautiful and perfect until she become horny and wants to have sex with you as well.
Be careful as you'll get a dialogue indicating Mario Emma Sharing Route at this point. This particular choice, either you want to Share Emma with Mario or not, has some relations regarding whether you want to Arrest Mario or Betray Sofia in Sofia's Questline. Personally, if you choose to pick Emma Mario Sharing Route, you might as well Betray Sofia as Emma will angry if you arrest Mario at this point. But if you want to Arrest Mario later, then don't pick the Sharing Route dialogue.
In this case, I assume you want to continue corrupting Emma, then pick the Sharing Route dialogue and the next time you go to the Sauna tell her that Mario also want to fuck her as well. Then, go to Mario's Office and tell him that she's ready.
Wait until next week and go to the Sauna again and Mario will join in the fun. Lastly, go to his Office to finish the entire Mario Questline. You can get 2 new scenes with both Emma and Mario in his Office from this point on.