How long ago did Bernadette join Ancestry, and how long ago did she last log-in? Are her results linked to a tree (private or otherwise)? All this could be quite overwhelming for her, even if she is interested in her family tree. Or she might have no mystery herself, and know exactly how she fits into the tree (in or outside of wedlock), and might not be interested in/have time for connecting with you. So much else is going on at present in people's lives, even in the best of times! She might also have had her DNA uploaded by a third party (even if she has a separate account) for some reason. I have a friend who got people to test DNA, not all of them blood relatives to her, just out of her own curiosity.
One thing about the results is that maybe she is much closer to the siblings then anyone else. She could be a surviving child of one of the siblings (i.e. your father's cousin). Her genetic distance to the rest of you is therefore less, which might explain the high results in some of the matches.
