It seems like you are trying to convince yourself here, we all want to believe in the people we love and trust. Regardless of what his family say about his past relationships and how 'bad' they were, this is just one side of the story, there are two sides to everything (I wonder what his ex's would say)....his family will paint a rosy picture...be true to yourself and trust your gut feelings here, please dont let him take you for a mug while he is having his cake and eating it....once a cheater, always a cheater....a leopard never changes its spots Im afraid. Even though you say he has cheated on past partners 'but only when the relationship was breaking down anyway' does not make it right....that sounds like an excuse for his past behaviour.