You do know that Lisanna's 'death' scene was anime only... right? Lisanna's fate was never fully revealed in the manga. The scene where the berserk Elfman punches her wasn't in the manga.

The explanation of how she took the other Lisanna's place was also anime only. The manga only said the other Lisanna died falling from a cliff.

And heck... the anima doesn't need to be explained. Do remember Edo-Jellal, aka Mystogan, has been in Earthland for a while. He did save Wendy's life.