Max's parents talk about living in the house when they were growing up, which surely would make them more like brother and sister than a married couple.
When Max pulls the cord on the mirror and becomes invisible the first time, he cannot see that he is invisible. Yet, when he pulls the cord with Noah, he sees that he is invisible.
Max and Noah pull the cord together in order to become invisible at the same time. Yet there seems to be no point to this, as it is revealed anyone can pull the cord to turn off the invisibility; later, Zack even sends Max into the mirror without Max touching the cord.