My sister is a hoarder. She has a partner and two children (7 & 9), and lives in a small 2 bed house. The children share, and it won't be big enough for them both longer-term. We rarely go round to their house, partly because there is nowhere to sit (barely enough space for them really, let alone visitors). The children have a small table and chairs in the living room they use for eating and crafts; that is always accessible even if nothing else is.
There is a path just wide enough to walk through the living room (which is open plan with the kitchen, and the front door opens directly into it), and enough room to move around in the kitchen, though there's lots of stuff on worksurfaces and around the edge of the floor. The bathroom is relatively uncluttered. The children's bedroom has little floorspace anyway as it's so small and has two beds in, but the shelves and cupboard are overflowing. The adults' bedroom has stuff everywhere (just enough room to walk to the bed and window).
Some of it is the stereotypical things that might come in handy some day (e.g. stuff that can't be recycled but she is unwilling to throw it away), some is old clothes (both hers and children's) and cloth nappies, some is toys (they don't need that many, nor have space for them). I would never say there's such a thing as too many books though, and at least most of those are on shelves 
And some is temporarily stored for charity volunteering she does (and therefore changes regularly).
She is unwilling to get rid of things without getting money for them (though obviously they are not earning her anything while filling up her house, so I suspect this is somewhat of an excuse), so I have tried to help her sell it before, but only managed to get rid of a small proportion, and I'm sure more has accumulated since anyway, as the underlying issue has not been resolved.
They don't have the use of their dining table as it and the chairs are always covered in stuff. The coat cupboard is mouldy, though it's the sort that might be anyway (with no ventilation, on a poorly-insulated external wall), but it would be lower-risk if emptier.
The house is obviously more of a fire hazard than average (they don't smoke), but not dirty, just messy. There are various DIY bits that could be done more easily if the mess was cleared, but nothing very urgent, mostly cosmetic. They have had tradespeople in to fit a bathroom, and fix things when necessary.
Her children seem fairly well-adjusted. The older one is somewhat anxious, though that could be because he was parented less responsively than the younger one in the early years, and he seems to be growing out of it anyway. They have good relationships with friends at school etc, and their cousins. Obviously their friends don't visit the house, but they meet elsewhere, and since their school is some distance away and their peers don't live locally, it's unlikely they'd go round there anyway. She is otherwise pretty organised and does lots of activities with them (both in and out of the house), and cooks from scratch every day (don't know how; I couldn't stand cooking in her house!).
Her partner hates it but hasn't been able to make much difference. I've been told they could afford a bigger house but he doesn't want to move in case the new house ends up like this one.
She hates any of us (me, my sister, my mum, her partner) mentioning it, or even referring to it indirectly (e.g. saying "we can't go round to X's" when making plans). She knows it's an issue but doesn't seem willing/able to do anything about it at this stage.
I don't want to involve any authorities myself, as I'm sure they would make things worse, not better (certainly overall, even if not in that specific regard) and she might never forgive me anyway. So that would likely be a net negative. Plus the children don't seem to be suffering from it particularly so far.
I've read various websites on the topic, and there doesn't seem to be a great deal someone else can do about it. They seem to need to be ready to deal with it themselves.
Any advice from someone who has had a hoarding family member, or has been a hoarder themselves, and got over it? What would you have found helpful for someone to do/say? I don't want her to end up living alone with piles of newspapers when she's 60. And obviously it would be better for the whole family if it was resolved asap.