Would i still have to pay for Data?

My sister got an new iphone and she still has her old one. If i switch from my cell to her old iphone she use to have will i still have to pay the data package a month?

The phone doesn't matter, just the SIM card.

Both the phone number and the data plan are tied to the SIM card, not the phone.

If you want to keep your number, then yes, you still have to pay for data, just like before.
And even if that weren't the case, would you let your sister pay for your data packages?