Hopefully it's enough of a variation to be unlikely that someone in your or your sister's life could spot you girls. I would generally advise changing names entirely. Public knowledge could be kerosene on your relationship. I read sparsely through your threads, but I shall take her extreme behavior and unexpected childhood recollection, I offer a proportional conjecture, so caveat emptor.

Your son not only personifies the end of your sisterly relationship as she sees it, his existence changes how others see her. He represents a life path you've taken that she does not have equivalent access to due to her sexuality. While it's possible that childfree is a life choice for her, decided assuredly and independently of her sexuality, the reality is that a natural impregnation and genetic offspring of both parents is not something that is in the cards for a gay woman. Therefore, her choice to be childfree is seen differently from how yours would've been.

This is where her belief in your mutual sisterly childfree lifestyle comes in. Her belief acted as a defense mechanism for her, in which your independent choice for a childfree life, would have shown others that you determined a childfree lifestyle was superior for you. As a capable, heterosexual woman, you present a more convincing case to society. However, society is much more likely see her childfree life as an aging woman with constant inquiry, along with looks of pity and judgement. Your childhood promise was an ally and cover for her childfree choice.

Your son's existence shatters that defense mechanism, and he personifies a life path that you've deemed preferable. His presence is a constant reminder of a life path that she doesn't have equal access to, and comes with the reminder that society views her life path as unfortunate and pitiful. Alongside a childfree heterosexual sister, a gay childfree sister has a mutual defense against public scrutiny. When you two are out together, she envisions society seeing two older woman having fun independent of children. When your son is there, she envisions society seeing the image of a mother and son, contrasted by her presence as an old hag.

When she is out with your husband, at worst they may be mistaken as a couple. Yet it is more likely to not garner much attention. However, if she is out with both your husband and son, it's likely that she will be addressed as a mother. People may make inquiries about the child to her. In having to correct strangers, she will constantly having to endure followup inquiries or judgmental stares about why she herself is without man and child.

If this is correct, then your sister is suffering a common insecurity. This insecurity is especially more likely if she's been single for most of her adulthood. Confident single adults are not common, since it requires multiple desirable traits which therefore likely lead to relationships. As young adults age, and their peers transition into the next "normal" life stage, the contrast of their independent adult lifestyle becomes increasing clear. Thus they are increasingly likely to be judged, and it's natural for many people to be insecure about it. As she grows older, hopefully she will gain enough wisdom and confidence to withstand external judgement or care very little of it.

Whatever it is, it's likely a bigger deal in her mind than in reality.