You can register anyone as a "member" of a MG. Sometimes I have been attached as a member.
However, a MG, to start with, has to have at least eight members age three or above who are staying at a Disney Owned and Operated Resort.
That said, each MG event must have an attendance where both at least of the eight members, and at least 80% of the attending members must be staying on property.
For the OP. You have 8 people who are qualified and five who are not. You can register all 13 as members of the MG. However, any MG event you attend must have all eight members present (because of the first rule) and can have no more than two (for a total of ten people) from your brother's family as having more than two would mean the percentage of people at a Disney Owned and Operated REsort would be less than 80%.