HUGE congrats to you two soon to be marrieds and Sunny, speculating I can cope with, however, if we are to learn of, or more of OTMAs psychology we must examine that of their parents.
I don't see Alexandra as having had a childhood in which she shared much time with a peer group. I see a lonely, isolated childhood during which she had little choice than to keep "things". to herself. I see her as being a child who did as she was told, perhaps from fear that if she didn't, this new, significant other would be lost to her, as had her Mama and siblings. She would have carried this tendency to secrecy with her into adulthood simply as a way of being and passed it, unconsciously, to her children. To Nicholas, she could do no wrong and to her children she was just Mama. I feel it may have been possible that she bent the truth regarding Alexei's illness, perhaps using words like "delicate" or "different" in her referals of him and I think it very possible that she was in denial, the ramifications of this disease and how it could effect her WHOLE family must have been intolerable, more so if she held the secret belief that she was responsible for it. I feel that familial discussion about the "condition" was not encouraged and I feel quite certain that when those other family members who shared it died, within the IF, it was never voiced out loud that X or Y had died of haemaphilia so it may be possible that OTMA never made the connection. Another possibility could be that Alexandra told OTMA that Alexei's illness was not to be discussed with others but I feel that this would have been unnecessary. Because they were so closely enmeshed OTMA would undoubtedly have taken their cue from their mother. It is very likely that whilst it was all being played out in front of them, so detatched were they from the effects of the disease that they may have had little realisation of what it could do to them. It may have been that, unconsciously, Alexandra had sought to keep them emotionally immature-and perhaps seen as unmarriagable-as a way of keeping them safe.
This, of course, is all speculative and just MHO!!!