Being powerless to prevent a woman returning time and again to a man who physically abuses her can be overwhelmingly depressing, and more particularly when there are dc involved and she refuses to acknowledge the harm that she is inflicting on them.
Dsis has admitted that she told them that I am some type of drug user the first time I contacted CS 4 years ago Either your dsis is incredibly cunning or she's mouthed his words which suggest to me that he may have had some prior experience involving other dc.
After having ensured that you are speaking to an officer who is assigned to the police authority's domestic violence unit, voice your concerns, mention the lie your dsis said she told to CS about you, and ask to invoke Clare's Law as you are worried that your dsis is not the first woman he's abused. If he hasn't got any prior history I would expect the officer to reassure you immediately but if not, go ahead as the result may be illuminating www.telegraph.co.uk/women/womens-life/11369454/Clares-Law-Find-out-if-your-partner-has-a-history-of-abuse.html
When you call SS make sure you speak to either the social worker who is dealing/has dealt with the case or to their Team Manager and make it clear that you are aware of what your dsis may have told them about you, that you are not and never have been a drug user or alcohol abuser and that if they should be in any doubt about this fact you are willing to present yourself for testing.
It's a good idea for you to alert the dcs school(s) as they may have noticed some alteration in the dcs demeanours. However, as this is a matter of the utmost confidence, I would suggest you only address it with the Headteacher.
The message you will be conveying to one and all is that you have seen him out and about alone with the dc , that as your dsis's behaviour towards you has recently undergone a change you have reason to believe that she may be cohabiting or engaging in frequent contact with him again, and that the dc are at risk of witnessing violence in their home. Say also that your fear is that he'll kill your dsis and that if he does he may decide to kill the dc too.
Keep a small notebook for the specific purpose of noting the calls you make. Always elicit the name of whoever you're speaking to and record it with the time and content of the conversation you've had with them.
If I were you I'd be incandesent about that lie. I'd make sure everyone I spoke to was made aware of it and it would fuel my mission to protect the dc, no matter how many calls I'd have to make before action is taken.