Swimming on the Moon by Brian Conaghan

Twelve-year-old Anna’s parents are going through a rough patch, but Anna can’t let them split up. Not when it might mean living apart from Anto, her twin brother. Anto might be a boy, and he might not speak (except using Lego bricks), and he might carry a coat hanger about like it's his closest friend, but that doesn’t stop the two of them being like peas in a pod. It’s a twin thing, and nobody’s going to separate them.

So Anna hatches a plan: get the whole family on a plane to Italy. Her parents have always been happiest on holiday. How can they fail to fall back in love at a swanky hotel with an actual pool to swim in and everything! But when Anna discovers more about why her family has grown apart, suddenly a happy holiday in Italy seems about as likely as swimming on the moon …

Swimming on the Moon is also available on Audible.