Frankly, i'd say this relationship is the type where she realises how much whe helped her and she is so touched by this that at some point she'll go "Oh brother, i wanna thank you so much!" *spreads legs*. Most incest games present bro/sis incest this way. Or the blackmail route where this happens: Ohno, you little shit brother you blackmail me! I'm gonna so kill you *spreads legs anyway, only to...* :kissingheart: Oh such big penis, please spank me!"
These are the usual variants. I may have presented them in a simplified fashion