Why Brian won't be in the movie

Consider his character had "retired" and "came out of retirement" at a moment's notice involving situations far less crucial to family as it would be if Dom turned rogue, what possible reason can the writers possibly come up with for the crew to not contact Brian to let him know that his brother-in-law betrayed the family and what are the chances that Brian doesn't find out or chooses to not act but instead continues to drive his kids to school in his SUV?

I think the writers may have an opportunity here though to take advantage of the audience's expectation that Dom didn't really turn "bad" and that he let Brian in on the scheme so Brian wouldn't interfere and stay retired which can also keep him and Mia safe. However if Charlize Theron's character is suppose to be anybody of competence, the fact that Brian didn't immediately come after her and Dom should be a red flag.