Honestly? Based on what you have said on here and nothing else?
I will tell you what is going to happen. Your sister might get a visit. One visit from the first response team and then a letter saying they are taking no further action.
I then predict a spike in her cleaning and hosptial attendences.
Based on your posts, SS will do nothing. There is no case.
'the fact they are so ill so much signals there is a problem' No it doesnt and I suspect the friend of a friend was just being polite rather than telling you to stop over reacting. Are you seriously saying you think these children are going to be killed?
See, I would not hesitate to contact SS if I thought a child was in danger. But I would actually look at the situation properly and take advice.
She cleans a lot
The kids dont go out much
They get stomach bugs and colds.
I am flabberghasted TBH.
The cleaning is probably got a lot to do with anxiety related to her children getting sick a lot. They get sick and she takes them to hospital, a waste of resourses but shows concern for her children's health.
They dont go out a lot? Them and millions of other children for a whole host of reasons.
Re reading your last post - this sounds like vindictivness rather than real concern.