You will have made this decision based on the needs and wants of all three of you and on the details of the little one. If you are happy to proceed then the next step will be planning how to support your BD and then planning some more, as from experience it is often harder for them than anyone imagined. What I think I'm saying is both things are true and valid, you all want to do this and believe you can...and BC may find it hard.
You're also right that, as here, people on AUK often do tend to post because they want support.
We are 8 months into placement so I can't tell you 'how it turned out' for us yet. BD (9) was excited, enjoyed the first few weeks and then did start to resent everything about AD (5). We put in Quality 1:1 Time with Mum, extra hour with us before bedtime, got books by and about adoptive siblings....She is still emotionally struggling and it affects school life. Fortunately the LA have listened and are going to use some of the ASF to give her a course of 1:1 talking sessions with a post-adoption professional. I have reached out on social media and got her a couple of penfriends who are also older birth siblings.
So as parents you do what you can but, and this is only our experience, it has been much harder for BD than anyone imagined despite tons of prep by us.
As I said at the start, if this is right for your family, go and do it. And concurrently, be prepared, even though you don't yet know how it will go for BD.
I wish you all the best and hope my honesty has not made things worse!