Spoiler: Don’t read if you don’t want movie details.
Now in Signs I've heard that the "twist" is that they aren't aliens at all, ….

AH HA…got it. M Night Shyamalan stated that he did not want to get ‘locked’ into the formula of having to provide a twist (or flip) after UB and 6th S. Therefore no ‘twist’ in S. The aliens are UFO. The title is representative of crop circles as advance evidence of the aliens navigation for landing sites.. But “Signs’ is also a type of person. “There are two kinds of people”. Group One, who believes in simply good or bad luck. These believe we are each alone and therefore completely on our own with a 50/50 shot at a good outcome. Or Group Two, who see a hand.and greater power behind everything, and therefore think life is not just a collection of serendipitous or deadly coincidences but often interactions with a force beyond us IE: miracles or ‘Signs”.

The flashbacks are Gibson regaining the faith lost upon the death of his wife. It was not electrical synapses firing randomly when she died; she gave him a precognitive hint for the danger he would face…”Swing Away”. The son had asthma for a reason…”His lungs are closed”.

Gibson starts the movie as a bitter Group One type and ends by regaining his faith that there is purpose behind the universe.

The movie’s intent was not emphasis on the aliens rather focused on one family’s reaction during a mind-bending 48 hrs. They had to rely on whatever inner resources they were capable of (just as in UB and 6th S) and I feel that if Shyamalan’s does have a theme or style in all his films to date that would be it. A protagonist in angst facing personal demons and finally moving past disbelief to an acceptance which allows them to tap inner resources and fulfil their destiny. As regards ‘lame’ aliens I’ve seen many arguments concerning that topic. The primary complaint the use of H2O. Did not bother me as I see evidence of human’s standing/walking/working around catastrophically fatal substances all the time, (geologist walking around lava flows). Humans (just as Night’s aliens) adapt by avoiding direct contact.