I have a 10 yr old daughter and I know its cliche, but I think girls go through periods like this. I think some alone time with you is vital if you dont already do this. Also ( and it may seem a bit daggy) Guides is a fantastic option to introduce skills for being part of a team. Maybe a team sport, like netball or soccer?

I also have another thought, but really dont want to offend. Could she be a bit jealous of Coco? Its probably a bit hard to go from being an only child to having a new sister in the picture. I was raised as an only child and I was ( and still am, lol) a bit bossy.
When my mum remarried ( I was also 8) I was really mean and nasty to him. He was very patient and put up with all my cr@p, but I hated him at the time.

I wish you all the best.