You don't have kids, while that absolutely doesn't mean you shouldn't intervene if you see an issue it also means you haven't got a clue how hard having kids is! Don't care how informed you think you are, if you haven't got them then you don't know.
It is absolutely not neglectful for children to be a bit grubby in their own home. If I changed mine everytime they got a bit of banana, berry, avocado, hummous (notice healthy foods are bleeding messy) on them we'd get nothing else done.
They come home absolutely filthy from grannys house, not because she neglects them but exactly the opposite she does ALL the things. Walks in the woods, mud kitchens, crafts, baking, beach, etc.
Its not great for children to go to school dirty for obvious reasons and obviously they should be bathed/showered frequently. But being dirty inbetween times is just being a child (that doesn't have an OTT/neurotic mother).
Junk food... not great! How do you know its all the time? How often do you witness what she gives them? She may need support here, if she had a rubbish childhood then perhaps she has not been well educated on healthy eating. Many people aren't. And it helps not at all that huge amounts of marketing goes into pushing 'healthy' options that are absolutely loaded with sugar. It would be very very easy for you to cook some nutritional dishes and take them over or even do a shop or two... 'here, I know it sucks being pregnant and being so busy so thought this would help'. Fresh fruit/veg etc is also infinitely more expensive than a packet of biscuits and has a much shorter shelf life.
Teeth brushing... i really don't see how you can know this. My children brush their teeth morning and night, but noone but me/DH witness this! Once again, does she know that she should be taking very young children to the dentist? Does she know that milk at bedtime causes cavities (but would seem like a very sensible option to most)?
Can't believe you called a pregnant person lazy! If she has limited help from her partner then perhaps she is in survival mode. Growing a human while keeping 3 other small humans alive and working (even part time) is about as exhausting as life gets. Yeah, it sounds like she shouldn't of had a 4th, definitely not ideal, but what exactly do you expect her to do about that now?
Constant headlice is not great, but again does she know that you have to treat ALL the bedding, blankets, cushions etc? Does she have spare bedding or a tumble dryer to even make this doable?
What have you done to help? You will potentially be her sister in law. You are very very well placed to help! By all means involve SS if you really believe the children are suffering and she won't accept any help but that doesn't evolve you or your boyfriend of any responsibility. You are family. Help them!