Who totally exists. Tim Lanning should know better than to make throwaway lines like that.
Harper's mother is currently unnamed, though we can assume her last name is Harper. Although maybe she's real progressive and didn't take Lord Anthony Harper's last name. She used to sing Stark Nude Wenches songs to Harper to put him to sleep, which potentially explains a lot about Harper's personality. Or doesn't.
She's dead now, along with all of the rest of the Harper family except Titus and his sister. Betrayals and Demonpocalypses have that effect.