I'm not a 100% sure, but from what I can tell she some how lost her memory because of something traumatic that happened. Her mother and aunt had clarified this when they spoke about it and her mother changed their names so it helped in hiding her real identity. So she appears 'identical' to her own self, but has no recollection of it and it surprises others at first only to believe in this 'fake' identity that she believes.
We sort of can guess from the start that she had the kid with him, but we're not sure what happened that lead Rui Rui to be 'abandoned'. Pretty sure this will become clearer as we go like most of the plots, but us as the readers can only facepalm because we can sense this drama.
On to the other part, they don't have an issue with their memory themselves as you can see from the age of Rui Rui, it has been many years since she 'vanished' so we're lead to understand it's normal for them to forget for the most part since her entire persona is slightly different then she was before she lost her memory. She doesn't even have any memories pertaining to her original name as we can see.
Granted there is a lot they are throwing at us which is chaotic for a story, but I'm hoping as it progresses it makes sense @.@ otherwise it'll be a series I just wait until it's a lot further along to read or hunt down for a novel of it if it exists.
Uh......how did that happen?that she lost her memory?uh......its making me confused.....do they all have memory lost now?