Can someone help me with family problems please?

Hi im Miranda and i really love my family.But as everyone knows each family has its problems. I don't know were to start but i guess ill start like this. My dad and mom have been married for 19 years and there marriage is not that good. My dad has cheated on her twice, but i know hes cheating on her again with some girl he works with. My dad works alot and he can be gone for a week at a time. I love my dad and he is sick with a lot health problems. But i know he is cheating because i met her before and i saw a bra in his car and he always helps with her kids and a lot more reasons. When i was little he was always gone and my mother basically raised me and by knowing he is helping with other kids makes me feel like he has a second famiy. I have told my dad about the way i felt and i told i know hes cheating but all he does is deny it.We also just bought this house to. i really love my father but don't know what to do to get him to stop cheating. Can anyone help me please i don't know what to say to him.