While Sophie often shares photos with Samuel on social media, Arthur and his girlfriend, Lizzie Friend, keep their relationship a little more private.
The royal — who is 25th in line to the British throne, just behind his older brother — shared a photo of himself laughing along the shoreline.
Earlier this year, the two brothers made headlines when they attended Prince Harry and Meghan Markle's May 19 wedding alongside their parents, Sarah and Daniel Chatto.
'Many congratulations to Harry and Meghan. Wishing you all our love in the future,' Arthur captioned a photo from the event.
'I wish Meghan and Harry a lifetime of happiness and also want to share some of the sweet love I gathered up inside me with all of you !! So tag someone you 💓love💓 (romantic or plutonic) and let them know why they're special ✨'.
The grandsons of Princess Margaret are not shy of sharing their lives via Instagram, and regularly post updates on their lives.