Is my dad's nikaah valid with my step mum?

I was about 17 18 years old and I saw my dad and step mum having sex, well they were not actually having sex my step mum was holding my dad private part penis and it was erected and I can't remember exactly if I have seen the whole things or part but I saw it and can't remember exactly. Now I just looked and moved away. I didn't have no feelings, no lust, no desire for my dad I just peeped through and look through a small hole. I didn't have no desire or lust for my dad. I just look but after I felt little discharge because when you see stuff like that naturally your body release that liquid but I didn't have lust. Even when I see couple kissing or romancing I get that discharge. Actually when I was looking, I was frightened and I felt bad and my heart was beating fast and I was scared. I didn't have any lustful feelings for him. I just looked. I was not married and I am not sure if I was engaged. Is my dad's nikaah valid with my step mum?