He had sex... with my sister

Long story short, my boyfriend had sex with my sister before we were officially in a relationship. Apparently he was talking to her before me and then he met me. He didn't know we were sisters (so he says). He always told me that nothing happened between them and that they only texted. My sisters best friend told me that they had sex. I asked my sister about it and she confirmed it. He finally confessed now that he's caught. How would you feel? I'm distraught.