Need a good advice plz thank you?

ok me and my bf have been having really bad problems. We've been together for 4yrs. He recently cheated me with an neighbor of his. Now his mom and his dad love the neighbor the both parents know wat he did to me but regardless they invite her anyways and im not allowed to go to his house. the mom previously has gone shopping and stuff like that with the neighbor. the parents know im still in the picture because my bf has said to his parents were still together the point is I cant respect his parents anymore i really am hurt of their betrayal because i have been so nice to them and it seems that their ok wit the cheating. By the way his mom thinks i dont know anything of wat she has done like going out with the neighbor girl. I know it because i get along pretty good wit the cousin and i know the cousin is not lieying because i have seen them together i dont know wat to do????