So can anyone tell what the sister did for him to get trauma?

If you skim the earlier chapters, his birth was unplanned and caused a lot of mishap on his family and their finances.. family was basically set for life before but after Inhwee was born, they suffered a lot of setbacks and so the mother/daughter combo projected that onto Inhwee. Amongst other things they also treated him vastly different, making him hyper-independent at a young age. His sister also intervened in much of his life so I think that’s where the trauma begins