So in the Doctor Who episode The Wedding of River Song, the entire episode is an alternate version of the season opener, which means the Amy and Rory in the alternate universe haven't been through the events of season 6 yet. But in the alternate universe when Rory asks Amy what's going on, Amy tells him that River is their daughter. How does she know this if she hasn't been to all the adventures in season 6 (namely Demon's Run) yet? Does the show ever state that it's the older Amy and Rory that ended up there?
The Amy and Rory in the 5:02 universe aren't from the lakeside, they're relative to the Doctor in his timeline. As evidenced by Amy's drawing of the pirates that she met in "The Curse of the Black Spot", her knowledge of River's identity, and her memory of the Doctor using the footage of the silents in order to convince humanity to kill the silents on sight, she (and by extension, Rory) has lived up to "The God Complex" in her timeline. Furthermore, she grew up with a time crack in her wall, which has made her memory resistant to changes in the timeline.
Additionally, there are some suggestions that at the end of "The God Complex", the Doctor dropped Amy and Rory off on Earth sometime in early 2011 (before he was due to be assassinated at Lake Silencio). This is evidenced by some in-story clues that "Closing Time" takes place over the course of 18-21 April 2011 (such as a newspaper in the episode being dated 19 April 2011, and the Doctor saying the "tomorrow" is the day he dies (whih he says on the night of the 21st)). Since (post-"The God Complex") Amy and Rory appear in that episode, that would indicate that there are actually two versions of Amy and Rory (well, actually, it'd be two versions of Rory and only one version of Amy, plus her Flesh ganger) on Earth at the time; the pre-Series 6 version (Rory and Flesh-Amy) who receive the blue invitation and go to America, and the post-"TGC" version, who are living in their new house in London. Personally, I prefer this explanation, as it makes more sense as to why specific versions of Amy and Rory that are relative to the Doctor's timeline appeared in "The Wedding of River Song", as opposed to just saying that they were pulled in from wherever the were in time to an alternate universe centered around 22 April 2011.
Because Amy is special. She can remember when others can not. She remembers all of these things for the same reason that she could remember the Doctor on her wedding night, after Big Bang 2.0.
Also, "The Wedding of River Song" happened after "When a Good Man Goes to War", which is when they learn that River is their daughter.
It is stated that people in the alternate timeline do remember things... like Amy remembers about the Doctor.