TIL people are literally incapable of imagining a platonic/sibling relationship between a teenage boy and his sister-in-law.
As for the parents and grandparents, yes they were irresponsible and distant, and the MC hasn't outright forgiven them for it. Rather, he understands their (flawed) reasoning and realizes that he's done the same to others in his life. He's resolved himself to be better, and in the process fix the gaps that their upbringing had left in his brother.
What his parents did wasn't okay, but they did what they thought was correct. MC realizes that there's no point to being stuck in the past like he was before.