Selwyn family

Selwyn is the surname of a pure-blood wizarding family.

Dolores Umbridge claimed to be related to the Selwyns in 1997, lying about how she obtained Salazar Slytherin's Locket. Umbridge took it as a bribe from Mundungus Fletcher, but told Hermione Granger, who was disguised as Mafalda Hopkirk through use of Polyjuice Potion, that it was an old family heirloom. She was likely lying to "bolster her own pure-blood credentials" ; however, it is unclear whether Umbridge was lying about her relation to the Selwyns or simply her own blood status.

There is also a Death Eater who has the surname Selwyn who fought in the Second Wizarding War.

Etymology
Selwyn is an uncommon surname of Anglo-Saxon origin derived from the words sel, "castle", and wyn, "friend". It is also the name of a mountain range in the Canadian Rockies, and a Cambridge College.