3rd child or just two

So my partner and I are in a dilemma.

We’d love a third child but our first is autistic, he is 4, and amazing but a handful! potentially going to go to a special needs school as he’s unable to cope in mainstream. Does not really talk although he is progressing. I wouldn’t want to take away any much needed time for my son. Especially if we did actually have another autistic child. My second baby was a girl who’s very social, she loves her brother but he doesn’t like to interact much with her but she’s certainly helped with his communications! She still may be autistic herself as my partner is autistic but definitely not as noticeable. Time will tell I suppose! If our son does need care for the rest of his life, when we are gone, I don’t want to put it all on her shoulders. ☹️ what to do? Would it be selfish? I’m so stuck! Don’t want all of the weight on daughters shoulders but it would be immense if she had two siblings in need of care.