As you rightly pointed out, you were both kids when this happened. She was only 9 and probably was trying to figure out something she didn’t understand. You were only 6 and didn’t know what you were doing either. 6 and 9 year olds don’t know much about sex unless someone older has been inappropriate with them. The incident does make me wonder what happened to your sister.
Please consider that feeling ashamed and revolted now is putting adult feelings on what happened as a child. You don’t report having had those feelings then. All childhood sexual experimentation isn’t traumatic or traumatizing. From what you wrote (and only from what you wrote — there may be more to the story), I’d say your OCD now is a much greater concern. You may be assigning more to the incident than it warrants because of your current state of mind.
Since you have been diagnosed, I’m assuming you are talking with a mental health provider. If so, please take your letter and this response to your next session to talk it through. If you are not seeing a professional, please make an appointment. A professional can help you sort out why you are suffering from such anxiety that you are becoming increasingly symptomatic.
I wish you well.