Am I in the wrong?

Summer
My (now ex) of over a year accused me of cheating on him with my adopted brother because my ex's dad saw us 'holding hands' even though that never happened since I had a cigarette in one hand and my bag in the other. Oh and a bit of a back story he cheated on me with his ex. Am I honestly the one in the wrong? I dont know why his dad would say that to him or he made it up.