Hello 😊
I have a stepson. I conceived when he was 10, I had been in his life since he was 4. His mum had a baby about a year before me (her partner was relatively new on the scene... definitely less than 2 years before the baby arrived).
Myself and my husband always said that we would wait until he was about 8/9 to start trying so that he understood a little bit more. We didn't want him to feel like we were pushing him out. But at that time his mum was single too so we didn't expect a sibling on her side first.
We just made sure that he wasn't pushed out. He always said he didn't want a sibling... my husband explained to him that we love him, but we want another child too and that's an adult decision. We made sure that he knew a sibling was always a possibility so he wasn't completely blindsided.
When the baby arrived, we made sure that he & my husband still had one on one time like they always had. Every Friday night they would watch a movie or go to the cinema or bowling etc just them whilst I had a bath or cosy night in. We kept this up, but I just went to the bedroom with the baby instead. We made sure that he was still able to do all his extracurricular activities etc.
My son is 2 now, stepson 13 and we are expecting again - that news hit him like a bomb though. He definitely doesn't LIKE being a big brother but I think that's more so his age.
He definitely struggles with having toddlers at both homes but he has his own room etc he can go off to and tbh he's off out socialising now.
Best of luck!