The time [m]y wife let me fuck her sister

Since having our first kid it just hasn’t been there for my wife. We’ve tried plenty of times and she just isn’t able to get into it which has dried up our sex life. Her side of the family decided to take a week vacation. Hours before we got on the flight her sisters husband got called into work (dr on call) he’s constantly working which dried up their sex life as well. First few nights nothing out of the ordinary. Wife and I try but she just isn’t into it no matter what we try. Next day we are out on the lake all day so we get back and everyone crashes early except us three. (Our first date we were talking about what we are into I tell her red hair/blue eyes. First thing she says you can never meet my sister)