My niece what's to move out of her mothers she is 16 we live in Iowa can she say where she wants to live

No, Until she is 18 a court has to set custody. So if you want her to live with you, you would have to go to court and petition for custody. Otherwise, you can be in trouble for custodial interference.

Can a 16 yr old file for emancipation in Iowa so they can decide. What they want to do

Do you have a job that pays enough to fully support yourself on your own (without anyone else helping you, not crashing with friends.. getting money from friends or having a boyfriend support you)... as well as money to pay for the lawyers and court costs yourself?

Otherwise you have to wait another two years...