I'm envious when I see my sister having sex with her boyfriend all the time.

Relax, honey, your sister is a good person and very grateful, and so is her boyfriend, so if you want, you can have sex with both of them. You can have a dirty threesome because he'll definitely want to, and your sister doesn't seem to mind, so darling, you have his cock at your disposal, damn it! And let me tell you something, that guy is a natural stud, and I think he'll give you both a really good fuck...