I think Alexandra at least knew of stories about her brother in this direction. She may not have believed them, you are right, because she may have said that it was because of Ducky that these stories were being told, and not because of any underlying truth. If she did, ( and I find it easy to believe she might not have), then she perhaps disregarded it, whatever she thought privately. But, in theory ( as it is very likely she didn't believe such stories), she most likely would not have been as understanding as Ella. But I can see her believing it, and disregarding it, if need be, whatever her opinion would have been, had she considered it.