When Ezio takes his mother and sister to Villa, he tells his sister that mother "will come back to us". And throughout the game she just sits beside the bed. Since I only completed the story missions and glyphs, I wasn't 100% synced. Does being 100% synced treats the mother of Ezio? Or collecting all the feathers?
When you collect all of the feathers your mother will give you the Auditore Cape!... that is the only thing that happens.
Nothing else happens when you complete a 100% synced.
What Ezio's sister may refer to is a trauma from what happened to her.