I'm not at all surprised Isla is struggling. She has watched her mother on the verge of a breakdown, her half term has been spend in a car driving the length of the country, had a short visit with her Dad and is now back in school without any proper downtime. The kid is probably emotionally and physically knackered.
Sarah's narrative is the same every time he is away. Hates the navy, hates living down south, not the life she wanted blah, blah. Isla will have picked up on this. Imagine being a child who has spend her entire life in England but her own mum refuses to refer to the house they live in as home.
Sarah is a broken record. A woman on the edge. She either has to put her big girl pants on and accept this is the life she has signed up to for the next few years or move back to Scotland now whilst he finishes his service or take the hit of the pension and tell him he has to leave seeing as he apparently has the skills which will land him a much better paid job out of the navy.