Sexual Fixation in a Child

Well here it is, my frustration of the year. Take into account that my 4 year old foster daughter/second cousin is currently in play therapy because of this exact incident, among other things earlier this year. We have had the talk about appropriate behavior between she and my 3 year son, the "private" talk, and the "if you need to talk about this you come to me and we will discuss it in private talk". But here I am again dealing the best I can with a fully clothed sexual reenactment with my son, who prior to her moving in had never even heard the word sex. After the first incident I am about at the end of my rope. Every time I feel these things are improving, she proves me wrong. In addition to the this, there is an excessive amount of self masturbation in front of who ever and an oral fixation that you would not believe. There has been suspicion of sexual abuse or exposure of some kind and I have tried to be understanding but at this point I am bordering on, "is this sexual abuse to my son". I feel like nothing is working to get this under control and I don't feel as though it is being taken as seriously as it should be by DHS. If anyone has any advice at all I am open for suggestions. I mean obviously you do what you can for family but at the cost of my own child's innocence?