Why would my ex boyfriend accused me with my sister?

Me and my sister have the closest relationship out of all my sister. I can talk to her about anything. Situation happened where my ex spread false rumors to my family about me. My family believed him but my sister and my dad was the only ones on my side. He than said I was looking at my father a certain way and me and my sister had something going on. It's very sick to my stomach that be would say that to me.

Why would my ex boyfriend accused me with my sister?
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