When pervy brother watches his hot step sis Lacey fucking her boyfriend he gets so turned on that he starts a hot fuck session of their own that makes his lil sis squirt on his big cock!
My sister and I have sex and she similarly feels very happy and satisfied when my dick gets hard for her. It's proof I love her and think she's beautiful. I've learned a lot of family members feel this way about each other. It's really nice.