I believe he's saying that having two family members in one realm helps you during a rebellion. This is not true.
From the wiki:
Once the rebellion breaks out, all the nobles in the realm will have the opportunity to declare their stance as either a Rebel or a Loyalist. They can also choose neither side and remain neutral. Nobles in the underground are not automatically Rebels, they must still declare themselves as Rebels. Nobles who do not choose sides will be considered Neutral, and will not participate in battles between the Rebels and Loyalists. If two family members are in the realm, then only one of them will be allowed to choose sides. The second family member will be forced to remain neutral.
Source:
http://wiki.battlemaster.org/wiki/Rebellion
