Family issues: What should i do?

I'm a teen and i feel so depressed lately. So heres the story my mom was seeing this guy and i really disliked him. He was rude to me a number of times and my mom dragged(not literaly) me and my sister over to his house every holiday and i absolutely hated it. I told her on a number of times and he even threatened me to act like i was having fun. So i confronted him and my mom got really angry and threatened to send me to my dads house(but i dont wanna go). She made a deal with me that she wouldnt contact him and in return i would be a good kid. But she lied she asked me to get her phone and there was a text message from him. Tommorrow is his b'day and she lied to me she said she wasnt going anywhere and then she randomly gets a call from work to go to somewhere for the whole day and night which is on his bday. Sometimes i hear her on the phone with his friend which i would ume is contacting him i heard her talking about bday presents. She only likes him for his money. This is the 3rd boyfriend and she has already been propsed to twice after dad and her divorce. I dont know what to do i feel so depressed and the time and sometimes even think those unthinkable thoughts. Im pretty sure my brother hurts himself ive seen the scars and my sister is clueless and i dont wanna talk to anyone about it. Help!