I adopted DD and DS when they were 8 and 6, near 7 (now 14 and 13). I later adopted DD2 (their half sibling) three and a half years ago.
The dynamic between DS and DD1. DS was very protective and wanted to be in control of DD1. He saw himself as having to be the one in charge and was very over protective. He slightly resented us, despite our efforts, when DD1 started to like and look for physical contact as he felt his perceived role was crumbling. It meant that he would tell DD1 not to do what we said or to go against one of our rules, just so he could try and show that we weren't the ones she followed and he still had his previous role, which was hard for DD1.
They clashed a lot too, as he could be very controlling- trying to interfere in her friendships, telling us that she couldn't do certain things (when she both could and wanted to) and so on. It was a real struggle for him to let go of his perceived role and be a brother, not trying to be the parent. Equally, it was hard to get DD1 to listen to us initially as she would take in what we were saying, but ultimately look at DS to decide if it was okay to do what we said, and she didn't have much confidence in part because DS was saying she couldn't do that or whatever, through protectiveness. DS also struggled because he had more difficulty school wise, and the age gap was close enough for this to be seen- however he has come on amazingly and they are close enough for it not to be a divider between them, although it caused resentment at the time, sometimes.
They are both incredibly close (and close with DD2), DS is still very protective but purely in the normal big brother sense, not in a replacement parent sense which is a big achievement for us, and the bond they all have makes me smile.