I took an Art History class eight years ago and I remember seeing this painting, though sadly I recall very little about it... it was a survey course, so it didn't go into as much detail as I'd like.
The centre 'brother' is looking at some Latin that reads 'ET IN ARCADIA EGO' hence the title of the painting. There is much written about this phrase, with some thinking it an anagram meaning 'Begone! I conceal the mystery of God' (I TEGO ARCANA DEI). I have pondered this painting for many years and think that it is Sarah and perhaps her three sons.