Hi there,
I am a regular on here but have name-changed in case my Dsis comes on here and recognises me from other posts.
So, the background is that both my Dsis and I have 1 DC, hers is a boy and I have a DD, both 2 years old. I am 12 years older than my Dsis so have lots of friends with DC. Unfortunately my Dsis hasn't had an easy time of it (although some of it self-inflicted) so doesn't have any friends. But we are a big family (there are 5 of us kids) and we generally support each other.
However, a few things have come to light recently which have given me cause for concern where my DNephew is concerned.
Both of our DCs go to the same CM and are with her together on one day of the week (my DD is there 4 days - Monday to Thursday and DNephew is there Thursday and Friday). DD has been going there since she was 4 months old, DNephew has been going since 1 year, to let him socialise as Dsis doesn't work and doesn't go out so we were worried that he wasn't meeting other children. So my Mum offered to pay for the childcare.
I am good friends with CM, we have a common interest, my DD 
so we've become close over the time. So much so that she felt that she could confide in me about some issues that she's seen with my DN. Please don't misunderstand, my Dsis ADORES my DN but...
So, CM told me that recently, whenever he's turned up to her, he's been chewing gum. That's the first thing that concerns her. Obviously as soon as my Dsis goes, she takes it off of him. The CM has started giving him some of her food because my Dsis is not providing the best lunch for him. We went out last week because it was DD's birthday. CM had DN for the day so the four of us went to a local farm. In his lunchbox my Dsis had packed him Chocolate spread sandwich, 2 chocolate biscuits, a chocolate flake cake and had handed the CM a MASSIVE bar of Galaxy chocolate (the really huge one) saying that they two children should share it! 
Now, my DD doesn't have chocolate, but only because she's not really bothered about it. I wouldn't mind her having it in moderation but she really isn't bothered.
Another time, he had some of DD's couscous (the flavoured stuff, she sometimes has that with ham/cheese etc for lunch and loves it). DN loved it so the next day, Dsis just sent him with a whole packet, nothing else. Again CM gave him some of her food. He also takes bottles of coke and also Dr Pepper with him.
But the more serious concerns (that one of my other Dsis's told me) is that she apparently puts him in the bath and then goes downstairs to make a drink and will sometimes even watch the TV. And then the worst thing was that somehow, my DN managed to get out of his back garden and walk to a local park, 20 minutes away on his own. His Mum was absolutely mortified that it had happened but I'm not sure how it did.
Now, all of us are learning as parents, none of us are perfect (my DD ran out of Mothercare the other day into the Car Park when I turned my back for a second) but I've learnt so much from MN and also from talking to friends who have children. DSis hasn't done any of these things so maybe doesn't know any different, but my concern is that these things (the chewing gum, the bath, managing to get out) are all common sense things.
I talked to one of my best friends the other day about it and she is a mental health nurse. She's spent most of her career working with individuals where the effects of problem childhoods have impacted their whole lives and her worry is that where these are the things we know about, there may be other things that we don't know about going on in the house. She suggested speaking to Social Services to see if they can offer her some help, but there is no way that I would contact them. My Dsis would never forgive me and I certainly wouldn't be able to live with myself. She absolutely adores that little boy.
She's now PG again so I worry even more. My Dsis is morbidly obese, has been for a long time and has had several MCs. She is on hormone injections daily to try and keep this PG going and her entire focus is on her new baby. DN is getting forgotten (she's always been desperate for a girl). Plus, when she was PG with DN, she could barely walk by 30 weeks. She's much bigger this time and because of all the MCs, I wonder whether she will be bed-ridden for much of the PG, what happens to DN then? All of us work FT so couldn't care for him and their total income is £11k a year so they certainly couldn't pay for FT childcare.
She's a very sensitive soul and I really don't know how to approach her about these things without destroying our relationship. Between the CM and I we have agreed that she's going to start providing his lunches on the two days that he's with her, and tell my Dsis that Ofsted are due to come and do an inspection and that she is supposed to promote healthy eating, so she's trying to get all the children to eat the same. It's a white lie as she's only doing it for my DN, but at least he will have 2 healthy lunches a week.
I know that I probably sound bitchy, but I don't know how else to write it. I love my Dsis and my DN and I want to help her, but without patronising her. Any tips would be gratefully received. By the way, her DH has two boys from a previous relationship and is a good Dad, although he does moan all the time! He works full-time though and works hard so is not always there to see these things...
All help and advice gratefully received. I'm amazed if you've managed to get to the end of this!