Maybe " Sweetie Pie"is an English adaptation.
...oh I think it was. I can certainly see English terms cropping up as the girls spoke English to Alix. "Sweetie Pie was used briefly, then he became " My S. " for the most part .
I'm suspicious of "sweetie pie" -- it sounds more American than British to me. (Anyone overseas feel free to correct me!)
The only time I've seen "sweetie pie" used was in Marina Petrov's translation of Olga's 1913 diary. As far as I know, Olga's diary was written entirely in Russian, so "sweetie pie" was likely chosen by Ms. Petrov to represent a Russian endearment that didn't translate neatly into English -- perhaps
солнышко (solnishko), which is literally an affectionate term for "sun." IMO
solnishko is a good fit because it begins with S, and can be translated variously into English as
sunshine, dear, darling, sweetheart, etc.
Solnishko would also dovetail perfectly with the imperial family's other sun-based endearments such as Sunny, Sunbeam, and Golden One.