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

hello. I'm 13 and i like with my dad and stepmom. my stepmom is 24 and she's 7 months pregnant with her 1st baby (it's my dad's 5th child...i'm currently the youngest). During the past months, i fantasize about my mom and stuff. i tell my 16 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!