Can I get custody of my boyfriend's children from a previous marriage?

My boyfriend has a legal separation with his wife. I have been in the home caring for his children for 5 years now. The mother of my boyfriend's children is an alcoholic who has left her children unsupervised on a number of occasions. She sleeps all day and drinks when she is not sleeping. She sends the kids to school late, in the previous days clothes and does not make sure they have showered. the kids feed themselves and run around the trailer park doing whatever they want. She has gotten into a physical confrontation with her 13 year old daughter on two occasions. She lost her job due to absence and tardiness and has used taken controlled substance medications from her workplace that her patients have refused. She has been in rehab for drug and alcohol abuse and has been in a mental facility on two different occasions for suicidal thoughts. I would like to get the children to reside with the children's father and myself. Do I have any legal right to these children and their care?