Can I have 2 iPhone's on one data plan? (read details)?

[Bill S]

Ok so there's me, my mom and my sister on a family plan. My sis just got a xenon not too long ago, my mom just got an iPhone 3G S and she's paying that 30 bucks more a month for the data plan. Now my phone is past it's two years and I'm ready for an upgrade. If I get an iPhone 3G S will I have to pay an additional 30 bucks or can we just use the 30 we're paying now for two phones...basically I really want an iPhone and if we have to pay that much more I can't get it