It sounds like your son needs you right now. I personally wouldn’t in your situation and I say that as a parent of both an adopted and birth child. They were similar ages as you described.
we had a wonderful life the 3 of us. My birth child had 2 devoted parents and everything she could possibly want or need. She didn’t need to wait to get her needs met. She was so excited about becoming a big sister. Both my husband and I had worked with kids in care so knew it would be tough and decided to adopt.
it’s been far tougher than we could ever imagined. To meet the needs of 2 children, we need to parents at home. We both work part time. Money is tight, careers we worked hard for, gone.
my birth child is the most amazing child, she is kind, patient and tolerant of all kids with additional needs because from such a young age, she has learnt to adapt to living with someone with complex needs (none of which we knew about prior to adoption but became apparent in the years that followed). Her life has been turned upside down. Considering only my birth child, I feel immense guilt about the impact it has had on her.
an example, we went to a school friends house for a play date last week who had kids in the same class as my kids. The older ones were having a great time. The younger kids went upstairs to play in the bedroom. I felt really uncomfortable, told the parent that my son has additional needs and we should watch him. She said, sont worry, it will be fine, it was not fine. A few minutes later we hear the screaming as my son had her son on the ground hitting and punching him because the other child took out a toy sword (you know, the cheap plastic ones). I quickly grabbed him off the other child (who wasn’t hurt but terrified of course) and had to leave with both children. It wasn’t fair on my older child, she was having a lovely time but because of my younger one, she was no longer welcome.
family days out are dictating by my younger child. No point going somewhere he doesn’t want to go. That will just end up with the youngest on the floor screaming and crying. Embarrassing for the oldest!
the oldest never gets break from him. They go to the same school. Break times are hard for my youngest. The other children find my birth daughter when he is dysregulated because she is good at regulating him. With a fear of adults, the staff are helpless. I’ve told them so many times to call me and not to disturb her from her friends but it’s the other children. They are afraid of his meltdowns. It’s a lot of pressure on a child.
these are just our anecdotal stories which you asked for. My birth child is amazing and because of her, my adopted child is thriving. He adores her and she has been the most incredible role model for him. However, I know she will grow to resent me for bringing him into her idyllic life. I know I would if I was her.
point of this is the birth children don’t ask for any of it. It’s not their choice. They may say they want a sibling but they can’t make an informed choice so don’t let that sway you. Everything is impacted - their time with parents, their needs getting met, their experiences, friendships, time with wider family because of a decision their parents make. Please consider this.