MessyONE:

The LW is not talking about pain, she's talking about punishment.

No, the LW is talking about pain and understandable resentment:

"I am in my late 20s and have several younger siblings. When I was 10 years old, my father had an affair. I found out because my parents were fighting, and my mom decided to "get back" at my dad by telling me. It crushed me. None of my other siblings were either old enough to understand or even born yet. The next day, my mother told me to never tell anyone about it, that it was a family secret. My dad apologized to me. My parents went to counseling, reconciled, and are still together. I obeyed my mother and never told a soul. In my early 20s, I brought it up to my mom. She pretty much told me that I should have gotten over it by now, as she did. A few years later, I tried to bring it up again, and she rebuffed me. Is it wrong of me to be extremely angry with my mom for dragging me into that mess? Is it OK to be angry that they went to counseling and fixed themselves but forgot about the devastated 10-year-old at home? (Apparently, she is still not too concerned with me.) Am I right in not telling my siblings? "

Notice the phrases "crushed" "devastated" "angry" "she is still not too concerned with me" - The LW, rightly, has thought all these years her devastation didn't matter to mother (which apparently it doesn't). How devastating for a child's developing elf-esteem. Nowhere does she use the terms "punish" "expose" "revenge" or "get even."

"am I right in not telling my siblings?".

Notice, also that--contrary to how many posters have interpreted the letter, the LW never actually threatens to tell the siblings but wonders whether the LW is right in NOT telling (i.e., continuing to keep the opressive secret). Since when the LW was 10 she had siblings that were "too young to understand" or were born "shortly after" and the LW is now in his/her late 20s, presumably they are all adults now, not impressionable children as the LW was. The LW has every right to confide her long-standing feelings of having been misused with her natural confidantes. Counseling would be better, but the siblings are NOT off-limits. Nowhere does the LW say "I want to get back at the bitch."

I would actually advise the LW to discuss this with the nearest in age sibling to determine whether mother has been manipulating any of the other siblings. Mother might just be doing a number with their heads too. If she's not, well and good, but the LW does not owe mother lifelong silence (or father either for that matter since he didn't take action to help the devastated child beyond apologizing).