Given that several members of Sheila's birth family were academics it would be doing her a disservice to write her off as an airhead. We know that at a certain stage of development a child sees itself as being responsible for what is happening in its' world. It is likely that Sheila believed that it was her fault that June had been absent for a while. At 2/3 it may not have been deemed necessary to give an explanation, but if one was given, it's possible that June may have said that she was poorly because of what she went through to have Sheila. Innocent enough words, perhaps, but enough to make a once rejected child feel in danger of causing a second. It's not beyond the realms of possibility that June told her she must be a good girl for Mummy or she would be ill again. If this was so, most of her thoughts during her school life, especially at a boarding school, would have been taken up with worrying about whether or not her mother was still there and having been disruptive enough to get herself expelled and presumably sent home to find she was, she was probably told by her mother how bad she was, how ungrateful, how she would never make anything of herself and possibly the Ultimate, "I should have left you where you were, I should never have had you"
In the end I believe she lived her life complying with how her mother told her it would be and "She" became very lost in the process, but had she been given positive, instead of negative affirmation, there was enough academia in her genetics for her life to have been entirely different.