This is a tough one. There are undeniably red flags here (refusing to brush her daughters teeth for me being the main one). That is completely unacceptable.
The cleanliness/cats issue. That's an absolute no no. However, when I was diagnosed with a brain tumour I went to absolute shit and could not keep up with the housework while I was struggling to get my head round the diagnosis - is there a possibility she's struggling with something? Not making excuses, just trying to understand.
Some of the other stuff imo isn't as bad (jamas in house we also do, however we do leave the house a lot more than it seems they do, so it isn't as frequent).
Nursery attendance - my youngest DD started nursery in September 5 afternoons a week and she has been off SO many times as we have genuinely caught every single illness going, repeatedly, it's been an absolute joke! Hand foot and mouth, flu, diarrhoea, sickness bugs, covid (battling right now), various viruses, infections.
However as I say, DD goes every day so it's different from only twice a week. Still not sure that one is a major issue.
The weed. Is she actually smoking it in the house? Not going outside to smoke, away from your niece?
I can't bear the stuff personally. But there are a lot of social workers who do not care one jot about parents smoking weed. Others would take a harder line. Ultimately it comes down to whether it affects her parenting, that's what they look at.
If you do report this, it could end up being a really helpful thing for her and her daughter. It could also culminate in potential removal (home conditions alone are something SS take very seriously and changes would need to be made, if it's as bad as you say).
So she could get reported, get a kick up the arse, make changes, and everything would be rosy.
Or she could get reported, refuse to make any changes, and it could then potentially escalate.
It all really depends on whether you feel your niece is actually in danger?
You know better than any of us.
It's a very sorry situation regardless.