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

hello. I'm 14 and i live with my dad and stepmom. my stepmom is 25 and she's 6 months pregnant with her 1st baby (it's my dad's 3rd child...i'm currently the youngest). During the past months, i fantasize about my stepmom and stuff...thinking about her growing and belly. i tell my 17 year old brother that when i i always think about my stepmom. he just laughs and tells me that i'm just jealous that a new baby's coming, and i'll no longer be the baby in the family. i'm so confused...help!