First off, let's break down this royal (and royal-tangential) family tree for those in the back: Pippa Middleton, sister to duchess Kate Middleton, is married to James Matthews. James has a brother, Spencer Matthews, who has gotten to know the Middleton family by way of Pippa.
And it seems he and James Middleton (Kate and Pippa's dog-loving brother) are wedding-going besties.