Bro, people keep saying that there are people fighting his ass but like I have only seen people shitting on him here.
What I will say is that I like the ambiguousness (I am probably using this word wrong but it's what came to mind so oh well) of his character. Like, I'm not saying I like him as a person. He felt like it was entirely his fault for the loss of his sister and he didn't seem to have anyone to help him through his grief even though he was a kid and to top it all of his dad comes home with some random stranger they have never met and without discussing anything says she's their new sibling. I can only imagine how flabbergasted and taken aback the brothers were (not that their responses where ok), it probably did feel like their father was trying to replace their sister.
Derrick never saw Penelope has his sister. His guilt regarding Yvonne and his anger towards his father's response to their loss caused him to lash out. But also made him never seen Penelope as his sister really. She was some random stranger in his house trying to take the place of someone else he dearly cared for. From personal experience this can make an individual feel fake and make it hard to get along with them, it's frustrating to watch them take over a lost loved one instead of finding a new place for themselves in the family, though I know that can be hard and that Penelope was not trying to replace someone though maybe hope to fill an empty slot. This is probably why Derrick has complicated feelings towards her. She was never his family though he acknowledged her as such due to obligation and didn't think much of it so when she got older and he knew her more she was a woman, not his sister, that he was seeing.
I feel like if it weren't for fake Yvonne Derrick would have treated Penelope better than before but only slightly and at most it would take a long time before he started to really think about things. I am curious to know what would have happened otherwise and if there would have been some other major event or something small that has a major impact that makes him realize things.
Regardless I think Derrick was neglected in terms of his trauma by his father and probably everyone else and ended up growing up shaping himself around his childhood guilt. None of this makes the things he has done ok, I would just like to make clear.