The guy's imagination is not taken away. Waited for the girls to watch a horror movie and then came over and fucked each one in turn. When you wake up and see a mask, you involuntarily increase your state of fear. And this adds a twist to sex, more hormones are released, including adrenaline. it is possible that such tricks he and his sister and girlfriend will do regularly.
The guy is lucky with his sister - she is a nipple. She's ready to open her mouth for him to stick it in her. Apparently she serves him regularly, because he no longer feels affection for her, but fucks her like a street whore - rough and bold. However, she seems to like this treatment.