What Are Siblings For

So Nia has always been Art's annoying little step sister. Their parents had just got married and both families had just moved in together and Nia turned up her annoyance. So their parents had just stepped out and Art was trying to watch TV and here comes Nia. This time she wasn't really being annoying but more flirting and trying to get into something I believe she been wanting to do for a minute. She started rubbing Art's dick through his jeans and Art was not ok with it but he wasn't stopping her. She proposed she wouldn't tell their parents or anyone per say. So she pulled his dick out and started sucking it and Art didn't stop her. Next thing you know these two step siblings were sucking and fucking in the living room. Hey what are Siblings For?