Fucking [M]y ex g[F]’s married older sister

Preface:
I was with my ex for around 7 years, from my early 20’s. Over that time I got close with her family, and her older sister Laura. Laura was already married when I got with my ex, and her husband worked shifts, When they had their son my ex and I wound up doing a lot of babysitting.
Laura was about 5”7, with a moderately slim build, long brunette hair which she always wore wavy and loved to wear fitted dresses. She was always touchy with me, and often gave me hugs which lingered a second too long, she also liked to bend over an ottoman to watch TV, which she did in front of me a lot. She must have felt my boner in one of our hugs more than once. She had crept into my fantasies one or two times.
Not long after my ex and I broke up, Laura become stuck for a babysitter, her husband was working a shift and couldn’t change, and Laura was due a well deserved night out with her friends. As my ex had moved away & I didn’t live too far, she asked me. I thought in the very least it would be nice to catch up when she got home.