Heck! . . . after 20 years you would like to think you could be all done with this lot!
May I ask . . . Are you sure of the real reason for her wanting to return? . . . is it really a sentimental desire for old friends?
It could be some thing else perhaps . . . maybe the recent natural disasters around the world? . . . or the political turmoil?
I just feel you need to be sure of her true motivation before you can sucessfully address it.
Maybe you can gently feel her out about it . . .