he Cheated with HIS SISTERS

I’ve been in this relationship for a long time. Years. My best friends are his sister, and step sister.... yea so I thought. After a 3 year relationship, dating since we were 15. I get a text from his step sister saying she need me to call her. So I did. Turns out. He’s been cheating on me the ENTIRE RELATIONSHIP with his FULL BLOOD SISTER, FULL BLOOD BROTHER, AND STEP SISTER! Like what in the bibity- bobity- bama is this? All four of them are all together; a frickin four way couple. They’ve been doing this they were 13! I have no idea what to do. I did tell their parents however.