This is just how I personally got it, might not be accurate: Big bro had noticed since long before the "rape" (in their words) incident that his brother was in love with him and wanted more than a brother relationship. He knew about him masturbating to him and remember the event when he was "dead drunk" where the younger bro joined in the sex.
So he sort of manipulated him by trying to drive him mad enough to force himself on him. And by running away he (in my opinion) took a gamble, where the bro would either give up or have his twisted love for him amplified to the point he would absolutely /need/ him and make sure he his (therefore keeping his younger bro to himself too?).
I think the epilogue went for a "He planed to have his brother and made him snap to do so" kind of way? Where he's actually just a twisted as the younger brother is.
damn
Can anyone just tell me what happened in last chapter because i don't get it.....