Saw Older Sister Get Taken By Sex Creep Of My Sixth Form (college))

My older sister was always a pawg with long brown hair and was always the respectful but great looking one.
Problem is during my college years before university I had a friend who was the known sex addict. He was always devilishly horny and went hard when he saw my sister pick me up one day from college. He claimed he could get my sister to bend over and let him fuck her brains out but she was always the respectful none slutty type.
One day I came out after finishing college to find her car not there. After searching for at least 15 minutes I found her car down a country path, and in her open door was my sister's bare ass was present and there was my friend fucking her hard. She was clearly enjoying it and moaning hard. I admit I loved watching her large ass cheeks ripple and clap with each of his thrusts until he finally came all over her ass cheeks and she began to stand up with her cum covered ass still bare.
To avoid being seen I ran back to my pickup spot and waited for 12 minutes until she arrived yet acted like nothing had happened.
Me and him were both 18 at the time and tbh I loved watching the class weirdo fuck my sexy sister. But tbh I don't understand why she let him