Don'tbuymesocks are you a single adopter? Because that's a reason to adopt that SWs understand, same as same-sex couples.
A fertile couple choosing to adopt will be questioned closely about their motives, as SWs tend to be wary of those wishing to 'rescue' a child. Adopted children are like all children- ungrateful little shits sometimes, and the 'but I rescued you!' dynamic is rarely a healthy one long term.
OP you wouldn't be 'taking away' from others, each family are assessed on their own merits. If you were the right family, you'd be matched with that child. However, as adopters with birth children, unless you have a special skill, you would be down the preference list. The adoption process is hard on everyone, especially children, are you prepared to have your birth children involved in the process, and then waiting for a child that may never come? I ask again, are you and your birth children prepared for a child with special needs? I have adopted DC and birth DC, and it's hard and wonderful in equal measure.
I think adding a child with the needs adopted children have to your birth children when there is no need to, i.e. you could have a birth child if you want a bigger family, is something that should only be considered when your birth children are sufficiently old enough to have the resilience to cope with the adopted child's needs. It could all be a fairy tale, but the common scenario is that it's not, and actively choosing that scenario is different to having a birth child.
I don't think I'm giving you a hard time, but I'm not going to say 'Yes! Of course, adopt siblings with a toddler and another young child already!' because that's a big thing to think about doing. I'm sure people have done it, but you need to prioritise your BCs needs at this age.