She has basically assumed all responsibility upon herself and offered him a guilt-free pass to love her. He has been graciously granted the right to use the excuse that he is saving lives by committing incest.
A smart brother would gratefully make use of the opportunity she has given him.
If there was any hope of avoiding the incest route, I'm afraid the dude has hit the 'point-of-no-return' by reciprocating her kiss. Now she's definitely not going to let him go. Better buckle up, onii-chan.
Big bro is just in denial. He's just using her imouto's words as an excuse to go out with her. He's subconsciously truly happy that he cried. those tears are of joy.