If I took over payments on a vehicle and paid the rest of the loan and the previous owner never signed the title over, can they take the truck back?

My sister-in-law, Ashlea, bought a $35,000 truck with her boyfriend, Brandon, in 2006. She couldn't buy it by herself, so she had to have him co-sign on it for her and his name is first on the title. They split up shortly after and she kept the truck. 6 months or so later, she got married to this guy, Kris, who was in the Air Force. Days after they got married, they had to move to Germany. Ashlea couldn't sell the truck so she told her sister, Lenzie (my wife), that she could have the truck if she (Lenzie) just took over the payments and she (Ashlea) would sign the title over to her (Lenzie) when ever she (Ashlea) got back to the states. Lenzie and I have made every payment on the truck since the spring of 2007 and we paid it off in May of 2011. Ashlea had only made a few payments on the truck before she got married and moved to Germany and my wife and I have paid the rest of the payments. Ashlea has never signed the title over to us and is now threatening that she is going to