First will state that I thought it would have been more of a tearjearker if Mia was the one murdered instead of Letty. Then you would have had both guys emotionally involved rather than just Dom.
Question is regarding why no mention of parents ... not only that of Dom/Mia but of Letty's or Brians? Or was it discussed in the first movie?
Poop in his hand ... Poop in his hand
I think it would've been more devastating if Mia died too-but how would she die? And if she did, Brian and Dom wouldn't have much reason to stick together.
Mia serves as the glue between them.
About parents...in the first movie, The Toretto's father was discussed. Dom opens up to Brian before Brian takes Mia out on their first date. He tells Brian the story of how their father died.
It's rumored that there is a scene in Fast Five where Dom and Brian talk about their fathers together. According to a thread on the Fast Five board, Brian specifically talks about his father.