Should I keep my stepdad's secret?

My parents got divorced a few years ago. I spend 1/2 my time with my mom & stepdad and 1/2 my time with my dad & stepmom. My mom & stepdad are really mean to me. I told my dad all the things they used to do (screaming at me for no reason, teasing me for me being fat, make up false allegations about my dad) and he wrote them a letter telling them stop doing it or he'll take it to court. My stepdad then sat down with me and said he was telling me the 'truth' and that I should grow some and stop telling my dad everything. He told me he'd only tell me if I promised not to tell my dad that he told me. He told me I wouldn't be allowed into the house again if I told. He told me all sorts of things about how evil my dad is, how he never made a proper settlement with my mom, how he has been plotting to bankrupt my mom & stepdad for years so he can have custody of us etc. I'm not convinced even half of it is true, but it might be. Sounds more like a conspiracy theory to me. Should I talk to my dad about it or would I be breaching his trust?