I Saw Something I Should Not Have Seen

My sister was 11 at the time (Deceased now). I was 13 when I saw her sucking the penis of our retired neighbor and his wife was watching TV at the time. I just froze and they did not see me and I never mentioned it to her or anyone. I was only there for about 5 seconds when he obviously ejaculated in her mouth. Back then I was nieve and did not know about ejaculation and I did not see her spit anything out of her mouth so I can only assume the obvious as they did not have children.
I believe that she was groomed and more than one man was involved but I don't have proof as I never saw it happen again. I believe she would go to his place on a Saturday but I joined a football team so I never saw.
Things that I believe were signs would be occasionally as I'm riding to a late start for football I would see neighbouring men enter or exit his home and sometimes during the week after school my sister and I would walk past and these men would say Hi and my sister looked like she blushed when she smiled.
She would go up to the farmers markets held on the park close to our home and come back with what looked like new clothes or runners and tell us she was given them with some story attached. She never seemed to have a lot of money and she would save her allowance.
She had a good life and she became a successful business woman with great children and a loving husband but that sight has been etched in my brain for a long time.