In Father's Day, it seemed to be established that if a person sees or interacts with a version of themselves from the past/future, reapers appear and devour everything that doesn't belong in the timeline (and more, for good measure). So why doesn't this happen when Amy Pond speaks to her younger self at the beginning of The Big Bang from season 5?
Early in The Big Bang the Doctor explains that the entire universe has collapsed, and the tiny part they are living in is all that's left. This is why he feels safe in using the Vortex Manipulator to pop back and forth along his own timeline, because the universe is already collapsing so there isn't much worse damage he could do.
I had always assumed that the same was true for the Amy+Amelia combination. Normally that would result in the fabric of the universe tearing apart and bad things coming to clean up the mess, but the fabric of the universe had already torn to bits, and those bad things had never existed.
The events of Father's Day did not occur just because Rose interacted with her past self. It was because in doing so she weakened time to the point that it couldn't adapt to the changes she was imposing on it. She knew what was supposed to happen, and forced something else to happen. This in conjunction with time struggling to maintain two copies of the same person at different times caused the events of Father's Day to happen.
Meanwhile in Big Bang the events occurred as they "always had"; there was no alternative that was being changed. She was fulfilling an event, not changing the past. Therefore time was able to be maintained.
A fun alternative theory is that the cracks in the wall represent damage to the nature of time itself. It is possible that creatures which are sensitive to time, such as the ones seen in Father's Day, had already been affected/destroyed by the cracks, and therefore could not come to "clean up the damage". The universe of course promptly ceased to exist, cleaning up any mess her self-meeting may have caused.