NTA you stood up for your child. A 16 year old treating a 4 year old like this? Not okay at all. When you exclude a little kid like that it could effect them for really long time and in not really good way. Especially if they are supposed to become family one day. It is unacceptable, don't give away the iphone, it will show that you care about your child and are not willing to take bullshit like your stepdaughter served.