My (30F) boyfriend (31M) of 2 years is sleeping with his cousin (27F). My course of action?

Hi. I've literally just found out that my BF of 2 years has been sleeping with his cousin. I'm just lost for words. We live in the same house. I had no idea, but now I can see there have been a lot of tell tale signs along the way. I've been a fool. Do I need a lawyer? Where do I even start?<br><br>&#x200B;<br><br>UPDATE PLEASE SEE MY COMMENT UNDER THIS POST FOR UPDATE. x