The whole point of a "family" unlimited texting plan is that it's unlimited for the whole family, all our phones. I'm paying $29.99 for the family unlimited texting plan. I'm paying $20 for my iphone plan, $29 for my wife's blackberry unlimited data plan, and my daughter doesn't have a data plan. All three of us can text as much as we like, and the charge every month is $29.99, and it appears under my line, yes, the iphone's line.
So, perhaps someone has messed up somewhere, but this is how it's been working for me.