My mother's like that, but since it was what I was used to, I didn't realize that much of what she did was abnormal/damaging until my mid-20s. My guess (based on LW's mother's response) is that the LW is focusing her pain/anger at the one obvious incident because she hasn't learned what other things were long-term harmful/not OK. I agree she needs counseling, so she knows what's healthy (like for relationships)...

Another thing to consider: some kids still really look up to one/both parents at age 10 as a role model. Finding out the person did something the kid was taught to view as truly horrible can be crushing, and can damage the kid's relations with the person long-term.