Wait wait..has your brother went to the cops about this? Also, according to you, someone tried to burn down the house with your brother's children in it and you are going to "wait to prove before I make my final conclusion", for something like that I would definately be at the police station so fast, forget waiting for the fire statement! The fact that you keep stating assumptions without actually going to the authorities is really weird.
Your SIL and your brother are sick people.