Dominic would probably try to go to his room and have a minute of quiet to calm himself down, then he would listen to some music or read (if his sister wasn't home) or go out to the main living area and make something tasty to eat with his little sister and have a movie night. They like animated movies and lots of things that revolve around robots, dystopian futures, and such.
If Cocoa noticed he was upset, she would go and ask if he wanted to talk it out, listen to him explain what was going on, give her best advice, and then back to the movie night XD.
Regardless, he's very extroverted and his go-to method for cheering up and just relaxing in general is spending time with his little sister who he loves to bits.
if his older sister was there it would go pretty similarly, except she's more aggressive with her advice, and it would probably end with them making a tasty dessert and taking or possibly playing a board game as they eat rather than watching a movie.
Honestly, their dynamic is just so sweet and wholesome. I really enjoy writing, thinking up, and designing the way they interact with each other XD