Am hoping to be able to give my sister some help / ideas on how to handle this. Will tell you what she says then add my two penneth at the end.
My neice won't eat. They have tried sticker charts, outright bribery and shouting but nothing works. She drinks a lot of milk but has 'no interest in food' and apparently will sit with her stomach rumbling before she eats, and then often won't.
My view is unfortunately this (but obviously this is all tricky to say, to varying degrees.) She does eat, but it seems to be mainly rubbish (biscuits, Quavers etc.) I have hinted this may be the case but told it most certainly is not.
The few times I have been there while she has been given her dinner it seemed to be either a tin of something (pasta chicken and sweetcorn) or a sandwich (latter is fair enough at lunchtime, of course.) I also suspect that food may be a control thing; they say they ignore it if she says she doesn't want her dinner but they actually do a lot of exaggerated sighs / pursed lips / tutting etc.
She is also, kind of affectionately, referred to as 'awkward' or 'a madam' and compared unfavourably to her 'easy' one year old sister. I raised the last two points with my mother though and she was adamant this would not be the case and that DN has never had an interest in food.
Raising the theories I have would result in lots of drama (no matter how tactful) so want to try and suggest constructively ways to help before the poor little bugger is sent to bed in disgrace yet again.