I think my pregnant stepmom is extremely beautiful. is this ok to think?

hello. I'm 16. my stepmom is 22 and she's 7 months pregnant with her 1st baby (it's my dad's 6th child in 4th relationship...i'm 4th child. my oldest step brother is 27). During the past months, i fantasize about my step mom and stuff. when i i always think about having my stepmom. i once accidently saw my dad 54 having with her and she was screaming loudly. after this incident only i started this. i always have temptation to watch them having and i have watched it a few times as well