Relationship report: my ex went after my sister

I met my boyfriend at the university and we were together throughout until I was posted for my youth service to another state.

Before the end of my service, I learned my boyfriend was dating my sister and he confirmed this when I called him to find out.

No matter how hard I try, I can’t understand why these two people, close to me, could betray me in this horrible way.
Can someone pls explain why this is happening?
Source Vanguard