Is that her? Is it x saying We’d be cross ( x has said that before, which made DD nervous to see me)
Who knows? It could be anything - it could be her friend, or DD could be punishing her father for bailing on her, or she could be feeling guilty about upsetting you/DH and reframing that as you being angry with her. Or she could just be a casual selfish teen who has something else to do that day and making excuses.
I’m sure she thinks it’s just a convenient place to stay while they see if they can get summer jobs.
Surely it means something to her too? It's her uncle's house and it was her grandmother's house. Is it painful that she wants to share your special space and hers with her friend?
DH says I should pop over there too. Should I?
What's the benefit? Will it achieve anything except to cause you more distress?
Could you visit your family house when DD isn't there, just to connect with bro and with your own memories? Or maybe take DS. You and bro could spend some time together and some time with DS, some calm relaxed time, even if it's too early to swim.
As for the family holiday, I wouldn't worry too much about DS being entertained. Yes it's only fair that there should be something he'll enjoy but if he's got his gadgets and wifi, he'll be fine.
I can’t compete DD is obv enjoying her new life.
It's not a competition. It's healthy for DD to be out having fun. Way better than sitting at home with her friend obsessing about their gender identities and jerking you and her father around. It sounds as if your brother's a good influence on DD and a good family connection for DD, less loaded than her relationship with you and DH.
You're doing amazingly well at such a difficult time, honestly!