I wouldn't call Lenka a diminutive, it is in fact somewhat pejorative. Lenochka, Lenok - these can qualify as diminutives. But really the girls will be called Alena (it is pronounced Alyona) and Lena, respectively. Alyona won't be called Lena as it is reserved for Elena.
I love Russian diminutives, almost all first names have them... although I never understood why Sonya stands for Sofya.

And why do some names have no accepted diminutives in spite of being long, like Nikita? Even Russians don't know :scratch3:

