I Think My Sister Is Having Sex With My Boyfriend

Just imagine my baby sister is sleeping with my boyfriend. I found out recently about their affair and had to travel to my village to confirm the rumor. I am a first year student and met my boyfriend when I was in form three. We have loved each other since then and we have been inseparable. He however did not perform well in his NCE and is still at home waiting for a college admission letter. Months ago he started visiting my home and would meet my sister and they would joke and laugh. I asked him about it and he said he was just playing around with his future sister-in-law. I was however told by my best friend that she has caught the two caressing at night near our home and this got me worried and I went home. I snooped through my sister’s phone and it was full of lovey dovey messages between the two. When I asked my boyfriend he said he loves me and all he wants is to sleep with my sister and dump her. I forgave him, but am now told they are still together and hanging around each other often. I do not know what to do I would dump my boyfriend but he is my first love and you know … this is my sister he is sleeping with. What do I do now?