NaPaTa has a thing for the younger sister seducing the older sister's man. It's a shame the older one never catches them in action and kicks them both to the curb, esp since the older sisters seem like decent people who don't deserve it.
Not really sure why a vanilla tag was slapped on this. If this is a vanilla cheating tale, then pretty the vast majority of cheating stories are vanilla. I mean, just because he said the often used words said by cheaters "I love you" doesn't exclude this from him convincing and then using the younger sister as easy pussy. If you love so much, why are you with her sister?