What adorable sisters! I especially liked the older one, juicy, mature. And she had a very good idea - to loosen up her little sister in this way, and not with a stranger from the street, whom one might be wary of, but offered her tried-and-true boyfriend. The older sister still needs to teach the younger one how to shave her pussy, either naked like her own, or to get a nicer intimate haircut.
Watching your wife suck other people's dicks is a buzz kill. And she understands that by licking other people's testicles, her husband's boner will get sharper. So these swinger couples swap to sharpen their senses, bring back the novelty, and make their orgasms brighter. Only I would make the lighting not so bright, then there would be more understatement and less embarrassment.
You could have given her a long-player so she could suck on it.