intolerable 18 yr old ss
So I'm new to this but I'm going to give er' my all 
so I've been in this relationship with a man with 2 teenage sons from a prior marriage for 4 yrs now. The 13 yr old isn't as bad but the 18 year is BEYOND out of control. He has called me a "raging c*n#" multiple times while his dad sat right there and said nothing. Its very disturbing that someone would allow their child to speak to an "elder" like that! My bf and I have a beautiful 19 mnth old toddler together and I love my bf but I REALLY don't want my toddler growing up seeing this disrespectful and rude behavior 
I'm not for sure because I avoid the 18 yr old as much as possible nowadays but I'm pretty sure he is doing mj as well. Stepson moves in and out a lot but he always comes back. I know its terrible to come between a father and his son but idk how much more I can take. Any advice?
Unless your SO is prepared to
Unless your SO is prepared to step up and lay down some boundaries for his adult son, then I wouldn't tolerate him living with you any longer. It is outrageous that his son calls you foul names and his father does nothing about it. This is no way to live. I would be inclined to give your SO an ultimatum, that either he grows a pair and tells his son to shape up or ship out; or that this is the end of your relationship.
No, it's NOT terrible to come between father and son. Especially when both of them are behaving like effing knob heads.
Who do you love more?
Who do you love more? Yourself and your child, or your SO? When you choose, you'll figure out the reset pretty quickly. Good luck.