Hey OP, I was this age when my siblings were born and I was the stepchild and also my child was this age when I had my second child (also was the stepchild)
My advice would be to make them the most important people that need to meet the baby first as they are siblings, my step GP’s also made a huge fuss of me becoming a sibling for the first time as did my family with my first born.
I was always included with discussions whilst my SM was pregnant and they let me choose outfits for the baby/blankets things like that etc so I did the same with my child.
My stepmum and my stepdad (when my mum then had a baby) both had conversations with me and told me that I am still very loved by them and my parents even though the baby was here and reassured me, and my partner did the same with my child.
I have let my first born be as involved with my youngest as they like, sometimes they wanted to give them a bottle or feed them breakfast as they got older.
You may also find as they get to be older teens they don’t want to do the family days out to the farm or zoo. I used to love the free house when I was 15 and my first born is also similar so I don’t push him unless they are birthday days out for example.
Finding family things to do for such different ages can be hard so I try to spend some 1-1 time with my teen doing things they like to do or my partner will take the oldest for dinner/ cinema, things like that.
Family time can be a game easier for the younger one but fun for the older ones or building forts/dens etc
I’m always grateful to my step parents as I always felt loved and still do to this day