I'm going to tell my in-laws how I really feel about their blatant favouritism
Why do you want to tell them off?
I don't know what's wrong with myself, but I hate both of them. I think they are disgusting. I wish I could keep the feelings deep inside me, never tell anyone about them.
I hear you, and I think it is ok to tell other people your honest truth, as long as it is respectful. You should take care of your own feelings, not just other people's.
I will never tell them my feelings. If they want to hear them, they should ask.
People don't normally ask your feelings about them unless they feel they are bothering you in some way. The only way they will know is if you tell them. Respectfully, of course.