Keeping a secret

What would you do if your boyfriend kept a secret from you for a year? My boyfriend just told me that he lied to me about who was staying with him? He told me that his sister who was mentally unstable was staying with him but he just confessed that it was his cousin. He said that he didn't want to tell me just in case I let it slip because his mom said of the family doesn't know. This person has caused a lot of fights between him and I. What would you do stay or leave?