I used to work for Sprint, and I did this all the time. What you do is you activate an individual plan, 69.99 for the any mobile any time everything data, and that's just your line. Then you activate a 4-line everything messaging family plan without data, which would be 119.99 per month (99.99 for the first two phones, 9.99 per line for the other two). Essentially you would have two plans under one account. Voila! It's much simpler than a lot of people think it is, and the phone reps are incapable of thinking outside the box.
EDIT: Thought it would be important to add this. While the above plan works for families with more lines of service, and not so well for those with fewer, you're at the point of equilibrium.
If you do the above, your monthly phone bill will be roughly $190 per month for 5 lines.
Putting your entire family on an everything data family plan would be about $190 per month for 5 lines (129.99 for the first two lines and 19.99 per line after that).
Believe it or not (and I hadn't thought of this before, I apologize), it's the the exact same to go for the family plan. It's also good protection against accidental internet charges. I've seen it happen more than once where a family member put their phone in their pocket and accidentally butt-dialed into the web, and got several hundred dollars in usage fees because they were paying per kilobyte.