Married Older Sister Fucked By Former Sex Creep Classmate

So bit of context my older sister is 37 and she was going down to Northampton to see an old work friend she hadnt seen but also i knew a former classmate from school called thomas who was infamous for being obsessed with sex and hitting on girls despite not being the most handsome of guys.

He was 26 at the time but during the evening i saw a message on Facebook where thomas had posted a picture of him and my sister in a selfie with a caption saying how you meet so many people when out on town.
I just thought it was a coincidence but later when i checked thomas Instagram there was a raunchy new video showing him fucking a woman from behind on his bed and the main indication it was my sister was the tattoo at the base of her back which was exactly what my older sister had.
The video ended with thomas Cumming all over her arse and my sister didnt come home til two days later and never said anything.
Im just stumped thinking why coz shes married and thomas was more of a weirdo than the most attractive of guys