I don't have BC but have adopted more than once so have gone through the worries about my first/second child and what will happen when the second/third child comes into the mix etc
My two girls were adopted as older children, but older than you are considering, and also with identified and diagnosed special needs, and my son was a younger child with no identified special needs meeting all developmental milestones. However I'm pretty unusual, most of the adoptive families I know have adopted children aged 0-6 without any identified moderate-severe special needs.
When it comes to children aged 4-6... the majority of the children do have emotional issues not present in a birth child. The majority, a few years along the line from coming home, are loved unconditionally and the families are happy. However even years down the line the majority need a bit of additional input. The most common issues I've seen families deal with are educational (not so much academic performance but social/anxieties/behavioural), social, and insecurities/attachment or trust related things. Those aren't issues which rule their lives and they are very happy and would not regret adoption for a minute, but they are additional things which are dealt with on top of all the standard things they deal with as a family.
Then there are a significant minority who do have definite special needs (emotional/behavioural) and another minority who don't appear to have any additional needs at all.
So my feeling is...realistically, in considering an older child, you need to be prepared and comfortable to parent a child with some additional needs which will be there for the majority of their childhood/into adulthood. I'm not talking severe problems, but things like a child who needs a bit more input in social situations, a child who needs some extra input in (mainstream) school, a child who can become quite anxious and then revert/regress back to some behaviours which test you, a child who does need time to attach. A child who does have some difficult behaviours although not so problematic they are needing to attend at CAHMS etc.
I would definitely reasonably expect you to be able to identify a child aged 4-6 who - has suffered trauma and has some insecurities/attachment issues/difficult but not scary behaviours, has some friends at school, is performing somewhere between a bit below to a bit above average, and is not significantly developmentally delayed or diagnosed with any moderate-severe special needs. Obviously though, it is a leap of faith and no one can predict how the move will affect the child etc.
Also, a child aged 6 who joins you is highly likely to have strong memories of their birth family home life and contact sessions etc, so be prepared also to deal with the topic of birth family coming up more often. Older children often have complex feelings about their birth family, it's a topic you need to be comfortable discussing.
However my children do have good sibling relationships and the majority of adopters I know with more than one child also do not have any significant problems with sibling relationships.
HTH