Sister In Law Watched Us

My wife's little sister, a 28 year old bum, has been living with my wife and I for about a year now. I used to tell my wife that if she didn't take care of the kids she'd be useless She has a nice body, nice supple tits and jiggly ass, looks great in a bikini, but her face isn't much to look at. I've never known her to have a bf or even attempt to get laid or be social.
A few weeks back we were hanging out in my shop late at night, and my wife finished her water bottle, looked at the trash can about 20 feet away and said "if I make it you clean the house." I said "if you miss you I get a bj." She missed by a lot. After some joking around and saying things likes where's my bj, she reached down, unzipped my pants and pulled my limp dick out right in front of her sister. She knelt down and started sucking. My cock instantly got hard as a rock, and sis in laws eyes were fixated on my cock.
I don't know if it was from her watching or if my wife just gave the best BJ ever that night, but it was amazing. She sucked me to completion, swallowed, then played with my dick for about another 10 minutes. My cock was about limp then she joked with her sister about tasting my cum and it sprang back into action. My wife asked if I liked that and I looked down at my dick and said, what do you think. She sucked me a little more but nothing more happened that night.
We did it again a few night later in our bedroom, and my wife was sucking away sis in law played with her tits, getting them close to my face, or letting me get a handful then backing away. I've been insanely busy with work since that day, but have a few days off coming up. I'm hoping to do it again, maybe soon sis in law will get a mouth (or pussy) full of my cock.