If you have a family plan, you have to have a family data plan if there is an iphone as any one of the phones. That's the position I was in... two smartphones and one basic phone, but you need the family data plan. Also, the smartphones will be charged an extra $10 month. So if you have ANY smartphones on a family plan, you have to pick one of the family data plans and add $10/mo for each smartphone.
If you have a family plan with multiple phones but they aren't all smartphones, you might want to look at ATT or Verizon since you pay for the data separately. See what the price is like... it's possible that it could be cheaper.
I can't wait for my contract to be up so I can move over to AT&T. I currently have a family messaging plan with 5 lines, but I want one of those lines to have a smartphone without switching every line to data. Not sure why Sprint doesn't allow this. They're about to lose customers left and right.