Hi syabunny11, and welcome to Scarleteen.
First of all, I'm so sorry to hear that this happened. You didn't deserve to have that done or said to you, as a young kid as well. It sounds totally invasive, confusing, and really upsetting. It sounds like it's taken some time to process what happened, and to come to terms with those feelings of violation and distress. Have you told anyone else about your experience? Has there been someone in your life that you've felt like you could trust to tell about it?
In terms of how old she was - that doesn't have much to do with the way that it made
you feel. Like you said, it's possible that that was something that she thought was normal because it was done to her, or she had been exposed to things that were inappropriate and invasive herself. There might be a whole lot of things that contributed to why she did what she did, but that doesn't change the fact that it hurt you and made you feel distressed.
You were a kid and deserved (and still deserve!) to feel safe, no matter who you were with (but especially people that you were living with or thought were a part of your family). You have every right to be upset with someone who has hurt you, regardless of their age or whether they "know better".
If you haven't read through it already, you might find it helpful to read through
Blinders Off: Getting a Good Look at Abuse and Assault, because it details why exactly things like that are considered abusive, and could potentially feel validating that you do, in fact, have a right to feel what you feel.
You talked about feeling like that moment "haunts" you, and stabs at you sometimes - is it making it hard to do things that you need to do in your daily life, like eat, sleep, go to school, or do your work? Is there anything that helps during times like this, like hobbies or activities that you like, or things that you think/say to yourself to get through those moments?
Childhood stuff can be so confusing and isolating to deal with - I'm glad that you felt safe enough to bring it up here. My shift is ending for the night, but I will be checking back in tomorrow and thinking of you in the meantime. <3