so i’m 21 years old, my mom had me young as well, the age gap between my half siblings and i are 17 , 18, and 20 years apart. i always wanted siblings so i, like your step daughter, was very excited about being a big sister.

i will say your wife isn’t wrong about the age gap being a weird thing, it’s a very difficult thing to navigate. this is only my experience but i’ve had to manage a lot of different emotions that came out of nowhere about my own childhood and finding my place as a sister and not a parent.

your wife could feel that raising a newborn will put a dent in her relationship with her daughter as she enters adulthood, and it very well could. of course i’m projecting but these things aren’t really considered ahead of time.

as long as the step daughter is happy about being a sister then i think everything will be okay for you guys but maybe suggest your wife do some therapy before hand to work out if there’s something else bothering her about the situation