Well, valeries right, but the way she explained it didnt do it justice.
The alien had hold of the boy, and used its poison gas on him as he held him, just then the dad had the flashback to when his wife died.
You saw his last conversation with her, but the important part was that at as she died she said "Tell Merril to swing away."
Jump back to the present, and with the whole "swing away" reference still in his mind, he looks across to his brother, who is standing right beside his baseball bat which is hung on the wall.
The dad tells the brother to get the bat, just then the little girl screams, distracting the alien, so the brother can rush forward and whack it with his bat.
Alien drops the kid, dad grabs his son and legs it out of the house with the girl, leaving the brother to kick the aliens ass.
Bro realises that some of the spilled water from a glass is burning the alian, so he has a quick glance and relises the room is full of glasses of water the little girl has left lying around, so he smashes them all over the alien, alien dies.
Meanwhile dad is outside, trying to revive his son, when he realises that due to the sons asthma attack, the poison gas shoudlnt have been adsorbed into his lungs.
So everything turns out fine, and so dad realises that because his son had asthma, it saved his life, and because his daughter had a thing about water, they were able to kill the alien, and the fact his wife told him something as she died that gave him the idea to get his brother to go to work on the alien with a baseball bat, he THEN realises that ......
.....there are no coincidences, everything happens for a reason, and there IS a God.