I need info on custody laws in TN?

I have been with my fiance for 6 years.He has four children from a previous marriage (ages: 12,10,8,7).They were married at 15 and had child @ 16.When we got together they had been seperated for almost 1 year, but were still living together.That changed immediately.For the first year she gave us problems.She called constantly on his phone wanting to know our every move.They got joint custudy with her having promary physical custody.We pay 600 mth in support.Now the problem:She is very unstable.Over the past 6 years there have been numerous suicide attempts.callsto pick up children b/c she can't control them, we take to all appts, and school functions.Cps has actually took the children and gave them to us twice.All of this and we can't get custody. We r very stable. No drugs, both have jobs, a home, i have no "biological" children,and we r commited to raising the kids in a better manner. The kids r having probs in school and it is out of hand. Need to know what to do to start.