You were probably triggered by your father's self-righteous posts on FB (it seems so, from the chronological description of events in your post).

It is one thing to let the sleeping dogs lie... back when your father kept quiet, he was tolerable

now that he posts self-righteous FB comments and behaves like an angel, you get triggered - the dogs are no longer sleeping.

This is a very difficult place to be in, especially since you stood up for an OLDER HALF-SIBLING.