How to deal with a controlling OCD stepdad (well mums boyfriend)?

my mothers boyfriend insists we call him our stepdad and constantly degrades my real father, yet if we defend him he will become even more rude, aggresive and even violent. I only visit my mum on weekends and i live with my dad for the rest of the time. My mum feels that she is financially dependent on him as the have purchased equal shares in an expensive house. Often she defends my brother and I but when we have left he wwill talk and manipulate my mum to see from his perspective, and occasionally she will take his side and tell us not to visit for a few weeks is this normal behaviour? In addition i am 17 and taking A levels i work hard and get all A's during the week yet on the weekend my brother and I are expected to work doing jobs such as stacking logs, moving furniture, the mowing etc. If we refuse he threatens and blackmails us, he also does little himself during the week and saves the main jobs for the weekend. he gets insecure when me and mum spend quality time together which is really nice, he tells dirty and uncomfortable jokes. I hate going there but i will miss by mum and even more the dog, she will never leave him despite she says she will so much that it seems like a daily phrase, my stepdad is irrational and will not listen and twists my words. Any ideas how to sort this situation now?