Is there another way that is better to explain this? My daughter is autistic, but as per many girls can mask like crazy and lives a (relatively) typical life - has some social difficulties, has a lot of anxiety and the typically teenage girl presentation of autistic with sensory challenges but she isn't violent, she does well at school and probably 99% of the people she sees would have no idea. So when I say she's 'autistic' without another word it doesn't feel quite right as the challenges are really different to people living with violence, pre-speaking, etc. My brother was diagnosed as 'Asperger's' and then 'high functioning autism' in different decades but neither are appropriate to say now. So just wondering if there's a better phrase you're aware of that I can try out with her...
Anyway... back to the point.
My daughter is autistic, as is my younger brother. So speaking as the sibling as someone with extra needs it was difficult, and it was nowhere as difficult as the situation you'd have for a theoretical new baby in this situation. My mother was wrapped up in my brother's care. I couldn't do brownies because he had therapy. and I still remember that. Am aware there's a balancing act for everything with two children (I have two myself), but there were many situations where my parents just weren't around because of some last minute emergency or similar.
I now have an autistic daughter and a younger NT son so am seeing some of this play out again in the opposite way. We didn't know before having our son, but with the sort of needs my daughter has it isn't a dangerous situation, just a slightly complicated one. I regularly think about how restricted my son's life can be at times - my daughter doesn't like noise and crowds so we naturally do quieter activities. I actually have no idea if my son would like a water park or a roller coaster or a massive concert as these things just aren't options as a family. Now that he's older we can and do split them up, but my daughter needs a 'lot' from me and I'm very aware that my son gets what's left. It's made him incredibly self sufficient and self possessed, and he seems happy, but I am sad that I'm not as close to him as I am to my daughter because I'm just really tired and don't have much left at the end of the day. And our situation is the best possible scenario - my daughter is mostly similar to NT children, just with lots of extra support at home, we have two parents and incomes, a decent sized home, lots of time and attention to share between them... and it's still hard.
So unless there's a longer term plan for residential care or similar for your older child, I'd be really hesitant to add a newborn to this mix. There are so many stories of autistic children who just can't cope with a new child and families having to split into different living arrangements, living with violence, etc. It's obvious that you love your child and want the best for them, but it's really difficult to see how this could work with all the extra surprises that come with a new child. Even the best case scenario is challenging sadly.