outline border

$color-primary: green;

.composition {
    //...
    &__photo {
        //...
        outline-offset : 2rem;

        &:hover {
            outline: 1.5rem solid $color-primary; //same as the border property
            //...
        }
    }
}

decreasing other siblings while increasing current hovered element.

    .composition__photo:hover {
        transform: scale(1.2);
    }

    .composition:hover .composition__photo:not(:hover) {
        transform : scale(.9);
    }