Wow, this is harsh.

Let's look at the mother's pattern of abuse (as far as we know by this limited glimpse into this family):

1. Vindictively humiliating father by exposing his transgression to his oldest child.

2. Next day, when heat of anger is over, talking to the child, minimizing the wrong done to the child by involving him/her, then forbidding the child ever to discuss the subject again.

3. The child is denied relief and forced to deal with her emotions alone and can't even unburden himself/herself by discussing the pain with a sibling. This is to protect MOTHER'S transgression (Mother gets therapy) as well as father's. Children shouldn't be held to the same standards of keeping secrets they are too young to deal with. True, the now adult child now wants a bit of revenge by telling her siblings, but let's face it--she's entitled. If discussing the situation she was put in with a close sister would help her, then confiding in a sibling should be her right. The mother had no right to put the burden of the secret on her child. (BTW, in my own situation I thought I was keeping mother's secret for two decades--9 years in estrangement of my parents--until my sister and I finally compared notes. It turns out sister knew more than I did and what I DIDN'T know hurt only ME. Mother was able to manipulate me precisely because I didn't know what my sister knew and I perceived my MOTHER as the victim and felt protective of HER. Yes, I met the definition of being parentized. Keeping the secret of mother's manipulativeness may actually be working against another sibling.) It would probably be wiser of this adult child to unburden herself to a therapist rather than a sibling, BUT, her siblings are undoubtedly adults now too (if she's now 28) and can handle a few ugly truths. This adult child has a right to choose either path to unburden himself/herself.

4. More than a decade later, when the child tries to discuss feelings she was impotent to discuss as a child, mother not only refuses to acknowledge her fault in putting a burden on her child the child was too young to handle, she blames the CHILD for still harboring resentment.

5. Almost a decade after that, when the adult child tries again to discuss the subject (and the subject is MANIPULATION, not ADULTERY), mom successfully uses the tried and true "blame the victim" again.

We only have a mere glimpse of how this mother handles adversity, but it isn't just one brief slip. To keep from admitting that what she did was wrong, she CONTINUED to minimize the child's distress, adding to the now adult child's feelings of injustice.