[Jords]

You dont have to directly say shes a buzz kill, that would be awful
Just try talking to her, asking whats up etc. The running away business sounds like shes attention seeking and might have something on her mind and want you lot to confront her. Shes in denial about being home-sick perhaps because she feels embarrassed about it. Try telling her you miss your family and she might feel at ease opening up, having a good cry, hugging it out and you have a fresh start because its all out and it can only go up from there. Get her to make the plans on what yall do 1 day.