subs in volleyball

Question
I am an vb official in a recrational power league
for a Sacramento Parks District in California. One
person has five teams in the league. Whever she has a player ill or absent, she feels she can pull a sub from any of her five teams. That gives her a pool of 25+ players as subs. Other teams in the league have maybe one sub or none. She feels we are singling her out because it is unfair to the other teams.  

Answer
Wally-
 Prior to the start of each and every season of volleyball I have ever played in recreationally, one thing has always been the same.  The captain of each team provides the league with the names of the members his team (the 6 starters), and names of any and all possible subs that the captain may use throughout the season.  Furthermore, any name listed as a player of one team can't sub for another, afterall, you're already a member of a TEAM! This required paperwork is to ensure that the captain has offically defined his team. I'm assuming that the fact that you're asking me this is because your league didn't require this seamingly obvious and important piece of paperwork.  There isn't anything in the offical rulebook regarding this, as you probably already know, and since the league didn't ask the captains/coaches to actually define their teams prior to the start of the season, I suppose she can sub whomever she wants, whenever.  
Food for thought?  Ask the teams to be defined before you start the season, along with subs, it makes things a little easier.  Hope this helps.  If I've misread your question, please shoot me back, I'd love to hear how this works out.  

Matt