When the WSM decided not to go to the pre-wedding parental brunch, why on earth did they try to cajole her to change her mind? I would have been privately celebrating her absence!
Not to mention that if they acted like they didn't care about her non-attendance, she would have likely done a 180 and INSISTED that she be there.
It sounds like the letter-writer really wanted her stepmother to be a mother, or at least motherly, to her (perhaps at her father's insistence?), and is truly disappointed, but she needs to face the fact that this woman has no family feeling towards her, and will not be a part of her life. Waiting for congratulations on her expectations is futile.