The Princess of Wales is celebrating her nephew Arthur Matthews’ 6th birthday today, marking a joyous occasion for the close-knit Middleton family. Kate has a strong bond with her younger siblings, Pippa Matthews, 41, and James Middleton, 37. As a doting aunt, the princess plays a significant role in the lives of her nieces and nephews, including Pippa’s three children and James’ son Inigo. The family gathering is expected to bring together the Middletons, showcasing their tight-knit relationships. Arthur Matthews, the birthday boy, is the eldest child of Pippa Matthews and her hedge fund mana…