These are some ideas of what you can do while you are thinking about it which might make a huge difference:
(a) take your neice out to the local parks, forests, beaches for a few hours every week for a runaround and long walk at least once a week, this will make a huge difference to her life. Try to get your SIL or your DB along and model for them how to play and engage with neice.
(b) take books and read them to your neice when you are there every week
(c) make food with your neice, both nice food (cakes, scones) and healthy food, take ingredients
(d) take your neice out regularly to whatever you have locally which is aimed at her age, so that could be zoos, theme parks, soft play, to watch classical music, art galleries, museums, talk with her in relation to everything you see
(e) as a present offer to pay for tennis for tots and offer to take neice or to go with SIL at first
Just having one adult away from home who cares, who they can talk to, who has their back, will make a difference to your neice as she grows up, even if you are not living with them. Uprooting a child or getting children services involvement, at the moment, can sometimes do more damage than good like others have explained.
I think you need to talk to SIL about the serious things, but without making her feel judged, so you could say "I was reading this article about the effect of screens on young people, especially violence" and lead this on to "it could affect long term mental health as she is too young to put it in context or process it as an adult would" and things about children who spend all their time on screens find it hard to learn in the conventional way, etc. That if she doesn't clean dd's teeth every day after just a couple of days gums will start bleeding when she does clean them etc. That if she were reported to social services, they would likely take a dim view of the smell of dope and smell of cats and so she does need to do something about that and you can help her. That dope can be smelt outside the flat, it is terrible for developing brains, and there are the dangers associated with buying it; I would tell her that if she does not stop it, then it increases the likelihood of her daughter being removed, so she needs to just completely stop it...
Your SIL and DB sound as though they could be "avoidant attachment" and this is likely to be related to their own upbringing and experiences in childhood - it isn't an excuse but it is a reason. Your sil sounds as though she is addicted to screens?
If you have limitless funds, input from a good psychologist as to changes needed for neices life, communicated to SIL and DB, and also some therapy for SIL and DB about intergenerational trauma and trauma and inspiring books on child development