William L. Shirer, The Rise and Fall of the Third Reich
The efforts of the Western Powers to produce a defensive alignment against Germany were well matched by the other side. Conversations between Ribbentrop and Ciano at Como at the beginning of May came to formal and public fruition in the so-called Pact of Steel, signed by the two Foreign Ministers in Berlin on May 22. This was the challenging answer to the flimsy British network of guarantees in Eastern Europe. Ciano in his diary records a conversation with Hitler at the time of the signature of this alliance.
Hitler states that he is well satisfied with the Pact, and confirms the fact that Mediterranean policy will be directed by Italy. He takes an interest in Albania, and is enthusiastic about our programme for making of Albania a stronghold which will inexorably dominate the Balkans.
Winston S. Churchill, The Gathering Storm
5/22/39 update at #8 - Ciano’s diary.
At this point, Unity Mitford would have still been on the scene, as well. So I wouldn't discount Sigrid completely.