We all in the state of FL, my brother and his wife have separated, not legaly but he has moved out 13 months ago, the mother who is 47 now has a 32 year old boyfriend who spends som noghts at her apt. My brother an his wife cannot speak to one another without there being an argument. My 15 year old niece is tired and want to live with me. Does she have the right to choose?

No. A child (under 18) has no right to make that decision. Only a court can award custody. However, if both parents agree they can give you temporary guardianship while they work out their differences.