Nonetheless, I cannot convince her to stop, and she persists in believing that if she "helps one more time" perhaps it will make the difference.

She also takes in and is devastated by my sister's rages, and yet she agrees with my sister that perhaps her life would improve if (god help us) she had a baby! So I guess my question is -- how can I help my mother with her denial, and with limit-setting?

A. It's understandable that you would be very concerned about this situation. Unfortunately, your mother and sister are both adults who make their own decisions. I would suggest that you continue to share your concerns with your mother and - if she's willing - come up with a strategy to use with your sister. This may include writing a letter to her and/or using some other means of letting her know what the consequences will be if this behavior occurs again. Introducing her to the book, "Stop Walking on Eggshells: Taking Your Life Back When Someone You Care About Has Borderline Personality Disorder" might also be a way of educating her about how to begin setting effective boundaries with someone with BPD. I agree with you that your sister's having a child at this stage would be both dangerous and counterproductive. She first needs to demonstrate good judgment and sustained control over her own emotions and behavior.