I am the only child of my father's current wife and have much older half-siblings. When I was little, one sister was very sweet with me; over the years, though, as her relationship with our father had ups and downs, she would stop speaking to him and therefore to me. The first time was when I was 7, and I didn't hear from her for five years.
It has happened periodically since. She has ignored most milestones in my life, and excluded me from hers, while not doing so with the other siblings.
When I asked why, she told me she could not separate her feelings about our father from her feelings toward me.
I am expecting my first child and am yet again disappointed by my sister's lack of acknowledgment. I would like to protect my child from her alternating warmth and hurtful indifference. Would it be inappropriate to keep her out of his life?
Answer: The best thing you can do for your son: Let go of the wounded 7-year-old girl. That nurturing big sister has long since been displaced by the sister who can't (won't?) get over herself. This is who she is. Start regarding any warmth from her as the exception, not the supposed-to-be rule.
As for your son, it's appropriate to protect him, but the auntie box-out seems needlessly extreme.
That's because, before you take any action at all, auntie's already of far less consequence to him than she was to you. From sister to aunt is a huge sphere-of-influence demotion unto itself - and she's a virtual no-show in your life. The figure who became a cruel tease to you will likely register only as a bit player to him.
Meanwhile, he'll take his cues from you. "Your aunt can't wait to see you" fuels completely different expectations than does "Your aunt may come, but we don't expect her."
By explaining her disappearances to your son honestly you can lessen the disappointments, equip your son to handle the ones that get through, and spare your family a new strain of the insidious "we're not speaking" disease.