Question concerning burial?

When my mother was young, she has always requested to be buried at the same burial site where her mother is buried. The site where her mother is buried is slowly sinking into the water. The church is located near the river. Very few members attend this little church, they are all elderly members, and no one sees after the ground. My mother is now 96 years old and she would not know where she is buried. My sister wants to honor her request, and is considering burying her at the little church. My mother already has a burial plot beside my father at another church and the burial ground is taken care of by the city. When my father ped, another sister left a plot for my mother, but at the time, my mother insisted that she wanted to be at the little church by the river. What would you do in a case such as this one?