I want to add to Steph's post and say it also depends on *your* situation as well. My oldest (7) has autism, my middle child (3 years 10 months) has Reactive Attachment Disorder, selective mutism, and significant social/emotional/behavior/processing issues, and my youngest (23 months) has selective IgA deficiency and FPIES but is neurotypical. All of them also have multiple food allergies. (The oldest is my biological son, the middle child was adopted from Vietnam as an infant, and the youngest is my biological daughter, and wasn't quite planned, but things turned out very well with her. :D ) Because I stay home full time, plus I have a background in working residential care for profoundly autistic adults, I am usually pretty good at keeping everyone safe, enriched, etc. I go out with them to the zoo, safari, store, library, etc. with very few problems, and everyone's safe (it takes a lot of extra planning, for sure). I absolutely could *not* do it if I had to work...I know that for a fact. There is absolutely *nothing* wrong with working...I just know that if I worked, I would have to find specialized childcare for the kids, plus specialized schooling for them, and find someone to take them to their various therapy and medical appointments and it would be impossible. If I didn't have background training in non-violent crisis intervention, I'm not sure I would have been able to handle the major meltdowns and rages, especially out of my 3 year old (well, last year...she's mellowed out quite a bit in the last 6 months).
The really interesting thing is that my 3 year old has *never* targeted her baby sister. She has targeted other young children, but never her own sister. Her sister is her best friend (her words, not mine) and they are very close...in age and emotionally. Having a baby sister is the best thing that ever happened to my 3 year old...having a typically developing sister has actually has helped her improve 20000% emotionally, socially, and behaviorally. I am not sure how that happened at all, but it did.
Our oldest never had a problem with either of his sisters as babies. As the 3 year old went from a baby to a toddler, he did have quite a few issues adjusting to the fact that this baby now moves around and takes his things. The two of them have such similar issues, that they are *always* butting heads. But I almost wonder if it's because he's 4 years older than she is, combined with the fact that emotionally and socially, he's still several years behind.
I will be honest and say that the biggest issue we have is that our neurotypical toddler spends a *lot* of time in therapy waiting rooms. Every Tuesday she is there for 4 hours straight while the other two have therapy sessions, for example. However, I do plan ahead and bring a bunch of things (a portable mini sensory bin, some small games and learning activities, drawing activities, books, etc.) so that she has her own fun things to stimulate and entertain her for those hours. (And the other issue is that she's immune compromised...we can't just rely on waiting room toys and activities even if they weren't boring to her because they are often fairly germy).
So, we balance it, and I think we do it quite well. But there are a lot of things that worked out in my favor so that I was able to make sure they all got the therapies and education they needed (the oldest goes to a montessori school and the younger two will be homeschooled), and I personally have the experience in an environment of multiple special needs individuals... For some families, it would absolutely.positively.NOT be a good idea to have children as close in age as mine, but for us, it works out really well. The first 6 months after a new child came home was really rough, but I think it is in every family to some extent. Now, things flow pretty seamlessly for the most part. There are some times where it's a bit chaotic or loud or crazy, but it works for us.
So, that's my long way of saying that even if you think your children can handle a sibling, you have to make sure that *you* can....I was in a therapist's office for my older two's therapy session a week after having a c-section with my youngest. The special needs life doesn't take a break. You have to make sure that you can still balance therapies with a new baby (although the newborn time was pretty easy for balancing...baby was in a sling sleeping or nursing through every session. LOL!). The more children you have, the more you have to balance. But it's entirelly possible depending on your own circumstances and personality (I am actually fairly decent dealing with stress and intense situations since that's what I used to do for a living...I'm pretty sure I'd be bored with zen-like calmness. ;) )