Why is my ex boyfriend friends with my sister on Snapchat?

Me and my boyfriend brokeup a while ago because he thought I was cheating to keep it short so he sent me a mass text saying how he knew it and so I texted him back and he left me on open so I unadded him. After awhile he just straight up blocked me but still keeps my sister as a friend on Snapchat.
Why is my ex boyfriend friends with my sister on Snapchat?
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