In terms of "second child syndrome", I think it depends on how much time your stepson lives at your house, as in this context it's more a matter of growing up with older siblings than biologically being the second born. But yeah, he would be your first born.
@Giorgia He is only with us two weekends a month (every other pattern) and 50/50 during school holidays so in that sense he will be an only child with an occasional sibling.. Of which to baby he won’t understand until he’s somewhat older.
I have a 4 year old step son but my daughter is my first. I do think she is the second child though generally as he is with us most of the time.
I have 2 teenage step daughters so I feel the age difference here makes it a bit more easy to differentiate between the first/second child thing! But I’d say your baby will always be your first child, for sure!! My step daughters have asked if they’ll be called “half sisters” and that broke my heart, so I said you aren’t half anything, your his big sisters and that’s all I will ever say! Xxx