fiance wanting custody

my fiance has a 3 year old daughter and gets her every other weekend and on wednesdays this was decided between him and his ex. She is 19, when he left he left her the trailer he bought them and only took his clothes. She had a job about 6 months and is jobless and carless with no drivers license! She relies on others for rides and now we have to take brooklyn ( 3 yr old) both ways which is about 20 mins. We dont mind so we can get her but she should be able to do one way! Also we have been having to pick her up or drop her off at the mothers friends houses and brooklyn has learned things no 3 year old should ( some sexual things) she also tells us she sleeps with her mom! She has her own room and bed at our house that she sleeps in! We know that she didnt pick up these things from us because we show no intamacy when she is in the house! We also know she smokes marijuana. She has scabies now for the seccond time! And is always sick when she comes to our house with a fever not just the normal winter sniffles. We need help the little girl is in a bad situation and we dont know what to do, she has also told him to give her tylenol to get her to sleep. Please help us!