i keep forgetting she's done awful things to me and a lot of people

On days where she "acts" like a mother, she cooks and cleans for us and says she enjoys it (I help her out a lot, okay).

And on other days, she tells us she'll spit in our food because of how ungrateful we are (partly because my stepfather did a mistake or my sister is being a spoiled brat beyond control).

It's been difficult to remember the latter usually when she's in a good "loving" mood. I forget the sinister things she does. I tend to open up during these times but when sh*t hits the fan, I just get hurt, regret, and blame myself as she often use them against me.

I've tried gray rocking her most of my life. But i'm afraid it had taken effect on me where I'm unable to react to anything. To jokes, to memes, to crazy stories from genuinely good friends or people. It made me a cage of my emotions.

It's starting to make me forget in general as well. I don't remember much of my childhood and the bad events now "don't exist" as she also doesn't remember them too.

Is this normal? Should I do anything to change this or is this reality for us?