My DD2 was nearly 11 and DS was 23 months old when I adopted him. I must firstly say that 7 years down the line I do not regret adopting my son at all. He is a joy to parent, a lovely little man and I just couldn't imagine life without him in it.
However, it hasn't been easy at all, and there are issues I never even considered
My DD2 has emotional, attachment and behavioural issues, and she seriously struggled with having another person in the house taking attention away from her. There were many many horrible days where everything was a struggle (it didn't help that I'm a single parent). It took DD a long time to attach to DS, the birth connection didn't help with that at all. There were days I even asked myself if I had done the right thing by adopting him or not
They do like the fact that they look alike, DD especially. But DD2 has a closer relationship with DD1 (who is also adopted but not biologically related) than she does with DS, and it's the same with him. The two younger kids love each other very much, but they also fight like crazy, they can really antagonise each other
The issue I didn't consider but should have, was contact and reunion. I always knew DD2 was likely to get into close contact with her birth mum, and I should have thought about what that would mean for DS given the 9 year age gap between them. When it finally happened, she was 16 and he was 7. He's really struggled with their reunion, he doesn't want contact or to have his birth mother close to his life, but he now has no choice as DD is talking on the phone to her a lot and visiting her and telling her all about DS's life
Basically, having two with competing needs has been hard, really hard at times. But as I said, I don't regret my choice. It's been worth it, because I have a wonderful bigger family like I always wanted, and I have a son who is the most amazing little boy ever. But the birth connection between them has if anything caused more problems than it has provided benefits.