Having step kids!!!!!!

I have 2 step sons and one is 8 he is the sweetest and me and his mom get along but I also have a almost 17 year old step son and me and his mom are on good terms but he is rude disrespectful and just plain out mean and I will get on to him and then he uses the your not my mom card but his mom tells me I can get discipline if needed which I can't I don't feel comfortable doing that but I'm 12w pregnant and can't stand to be around him but I don't wany my fiance to be upset I'm just to a breaking point and don't know how to handle him please any advice is appreciated how do I handle him?😪😭😥😩