The 'problem' here is lack of sufficient interest from the community to make up a diverse committee.
If there is 1 family (or other) discernable group that is proactive, community minded, outgoing and altruistic amongst a community where such characteristics are sparse, then the one group will naturally become dominant.
It is only a "problem" if the cohort act as a cartel - it is perfectly possible that they are all independently minded, free thinking individuals.
The dilemma would be to sack off certain members from a committee and leave it inquorate and unable to function or to retain a family group and function in what ever way it does.
The question should be prefaced by some detail about whether the situation creates a problem or not...
But in any case, the answer is no, there are no guidelines other than those which would qualify an individual for a committee role.
Was there a council vote to appoint members to committees? If so, and if there was a concern, then the vote should have been no....