I had trouble with this as I read the story also. I think it has a lot to do with that fact that Tita is destined to be with Mama Elena, no matter what. We all tend to take for granted those closest to us...perhaps Mama Elena does the same? In her mind, Tita will never leave...no matter how she is treated by Mama Elena...she will do the cooking, the cleaning, the bathing of her mother because she is the youngest daughter and that's what the youngest daughter does. She did not seem to care that Tita hated her for all she did and said...not until Tita told the ghost the truth and Mama Elena disappeared for good.
I think Mamá Elena recognize the love between Tita and Pedro from her younger days and is bitter for her own loss of love. And yet, with her fanatic hold on principles, she doesn’t see that Tita could experience what she missed. Or perhaps she’s just cruel, and doesn’t think Tita deserves what she didn’t get? It seems like Mamá Elena just sees Tita as “her daughter with her place in life” and not a human being as herself.
There is a obvious link between the development of Rosauras personality and Mamá Elena. Just that Rosauras personality is less characteristic than Mamá Elena with the lack of a love story.