Hello! We're so glad to have you here :) I'm Glenna and I only have Lily for now.
She and Amber would've been in the same year, and both prefects so they probably would've known each other. Lily's a little bit of a troublemaker though, so I don't know how Amber would've felt about her.
And then Lily and Dom would be cousins, so I think it's safe to say they know each other too, but like everyone else, it's up to you as to how well! Lily probably would've looked up to Dom, she was jealous of her older cousins and wanted to be them when she was younger. She's never been too fond of being one of the babies - always the last to do things; she's kindof impatient.