Responses

Hes 100% a creepy lil guy, but I think its showing how he attached his meaning of life onto her because she was nice to him (im assuming). No excuse. Like you said, even though they arnt blood related for all purposes she saw him as a brother and he should have respected that and been a GOOD big brother.
Is this chapter trying to excuse the incest? Like sike, they are not blood relatives ,it's not as disgusting as you thought?
Nonononononono ,they grew up together, he is still deranged, the situation is not better with this added context.
Same for the abuse ,just because he was also abused as a child does not make him any more sympathetic.