Dear Alexey,
Interesting -- Marina certainly isn't acarpous biologically, though her son apparently is.
But Ada -- there's a puzzlement. Is she sterile? There are certainly some hints that she was pregnant at some point, and I have wondered (but haven't gone beyond wondering) whether her relationship with Violet isn't maternal (or more likely grand-motherly). Van of course interprets her affection differently.
Marina claims that she and her brother had a rather conventional relationship, preferring to discuss mundane subjects as compared to the blinding, dazzling anyway, brilliance of Ada & Van's repartee. Perhaps Aqua was the brilliant one in that gemmed relationship. Their brother Ivan was certainly "above average."
On the other hand, there is the evidence of Marina's botanical diary. I might deduce that Marina's dur-nost' might be in the theatrical "dumb blonde" tradition. What color is her hair anyway?
Carolyn