September went back in 1985 and (not)saved Peter. This not only means that Peter himself was wiped away (since he died as a child), but every single Peter-dependent event post-1985 was physically altered up to the present (2011).
In the case of Olivia, her life in the rewritten timeline still physically happened (as did the lives of all the characters in this new history), but her memories of that very life are being overwritten by a different set of memories. So pretty soon, Olivia won't remember what has happened in her life, but other characters will (unless they start getting mind-wiped too).
Does this answer your question? As you also noted, the question you are trying to ask isn't quite clear.