What I don't get is why this woman doesn't think to herself 'F him, I'll show him I can care for her. I'll show him I can wash her hair and deal with her lice. Then he can go shove it'. Why doesn't she want to prove herself right, and prove us wrong? Why does she seemingly want to fuel our knowledge that she isn't caring for her daughter properly? It's always been the same, and I really, really don't get it.
Because she does not THINK like you.
It is never going to make sense to you or him or anyone. She is an alcoholic and a drug addict, yes? Their reasonings, rationale, and motivations, are not normal; they are focused on something altogether different than for those of us who do not drink or drug.
Having been exactly where you are with several children and several addicts, I truly understand and sympathize with you. It is a noble and caring thing to try to protect a child. Social services is the same in the US: Limited and frustrating. But let me ask you something. What have you so far been able to control? Is this child in danger of physical harm or threat to her life? Does she have a reasonable amount of food to eat? Of course your ex-partner's sister is not going to ignore you, that would put her at risk. I'm concerned that filing another formal complaint against her may get you nothing other than an angry mother who will make things more difficult for you and dad (which is what happened to me).
I am not saying you are wrong, and not telling you what to do, but I honestly suggest that if you want to help this child who is quite literally STUCK in this position, moreso than any spouse of an alcoholic, you and your husband will go to AlAnon and get some books on these topics and learn about the situation this young lady is in. It did me no good and actually backfired on me to focus on the alcoholic addict and her behavior. It has been years since I have seen the children I thought I needed to protect.