My niece is too sexual for her age?

Granted yes she has been messed with by her step dad which is why my sister and her two kids moved in with me. To escape that situation and so her ex husband doesn’t know where she is.

So, my niece did something really inappropriate with her privates (she’s 6) with my daughter watching (4!!!) and I am not okay with that shit. My sister denies it even happened but I know wtf it was when I saw it. Now my sister is upset with me. I pay all of the bills EVERYTHING for my sister and her two kids plus my daughter and myself. She does help me with my daughter by taking her to school, picking her up, etc. because I work 12hr days 5-6 days a week.

What do I even do about this situation? The goal when she moved in was for her to get some kind of job going for herself and start becoming a contributing member of society (she’s about to be 30) but it’s been almost a year and there hasn’t been any movement. I don’t want to ruin my relationship with my sister but something has to give and I have already had several talks about her daughters influence on my daughter as well as needing her help with bills and other expenses that I single-handedly pay myself while only making $18 an hour. Thoughts?
My niece is too sexual for her age?
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