I have thought about this quite a lot since I first read about it.
Since RSRD we know that both his parents are still alive. He tells Louie that 'mommy' and 'dad' were coming. He used those words to get through to his brother - it was most likely how they referred to their parents when they were younger. Their father was probably a very strict and demanding person and their mother a sweet woman.
The fact that he
did call them indicates that both brothers have not broken completely with either of their parents.
In Obsession (S03E13) Danny invites Hawkes over to eat at his mom's and not his parents', so perhaps they are divorced and he somehow blames his father for that.
He clearly didn't have a lot of contact with his brother. But I think that was because Louie kept him at a distance to protect him from the people he was hanging out with. Danny's reaction when his brother was admitted to the hospital was one of regret - too many wasted years.
So for now I've come to the conclusion that he has not been the victim of abuse. But perhaps he (as a child) was the
witness of abuse and wasn't able to do anything about it and he also noticed that nobody was helping the victim. That could be why he so often tries to protect people now.
from TV Guide: "Danny's challenged on a personal level because of his brother Louie (who may or may not be dead), and there's a deeper, darker problem. He just can't shake his previous life."
I guess/hope these are two different stories:
A) Louie is still alive
- he contacted Danny in Trapped because he needed help with something - now he's out of the hospital and still needs help with that.
B) The abuser or victim from his childhood has something to do with a new case - abuser still abusing? - victim became murderer?