Re: Need advice/opinions on problem with sister
Without knowing what she initially posted, I would also say it sounds like she is either jealous of your wife and family, or very insecure about her place in the family.
That said, to answer your question, a message board is nothing like a journal. She is making accusations/sharing feelings in a very public place where not only a few people can read them, but anyone with internet access.
Maybe your mom doesn't understand just what a board is, but she is wrong on this. Your sister slung mud very openly and publicly. Confront her on it.