Replying...
Intro. You deal from hyperspermia, a rare condition that makes you release sperm in order to treat it. Your four sisters and your mother help you treat it by letting you do what you please with their bodies whenever you want to or need to. This free-use arrangement is something that has been going on for a year now so it's completely normal now. You can use your mother or your sisters wherever and whenever you want to and they'll act like it's the most casual, normal thing ever. That's because it is normal now.

Your Free-use family

@Roman