The Most Revd. Justin Welby announced last week DNA evidence has shown 63-year-old Jane Hoare-Temple to be his half-sister.
He described the revelation his biological father was not Gavin Welby, the man who raised him, but the late Sir Anthony Montague Browne as "a complete surprise".
The Archbishop was conceived during a liason between Browne, Sir Winston Churchill's last private secretary, and his mother Jane Williams, who is now known as Lady Williams of Elvel, prior to her marriage to Gavin Welby.
Justin Welby is in Zambia until next Wednesday where he is attending an Anglican conference.
A friend of the Archbishop told The Daily Telegraph: "For him, the absolutely foremost thing in his mind will be his half-sister's welfare.
"He will be thinking about her emotions, her reactions and when would be the best time to contact her, unless of course she gets in touch first."