You can get extra family members relatively easily. Even if you sit back and do nothing its likely that your existing family member will occasionally offer to adopt people into the family. You can also try to force the issue by letting a captain take charge of an army for a few battles (eg: fighting against rebels) after a few wins or even just on good one you should get a 'man of the hour' event and you can then adopt that captain. Finally, you can always bribe your rivals family member, though if you want someone decent, that can cost a bit. Tried this with a Greek general one time, and he came over relatively cheaply. Of course he also developed the minion vice within two turns... Guess that was to be expected!