I have dealt with this situation on my first Section 8 property. I first attempted to bring in all parties of 'A' for a meeting to discuss all issues. I told them all what was going to happen if a workable, legal option was not achieved. The unwanted visitor stayed. At the time, working with and accompanied by the NET Officers, I served the grandparents an Order of Banishment for the granddaughter and they were told she could no longer visit. They would have to meet off property to pick up the great grandchildren but under NO CIRCUMSTANCES could their mother come onsite. Furthermore, they were now in danger of having the babies taken by CPS: mother smoking pots around the children and for the mother being homeless. Yes, I felt akin to social workers but in this case, I felt the greater good (my residents) comes before a single individual (an illegal occupant who has no right to visit.) The Police Officer also emphasized to the grandparents and their granddaughter that the granddaughter would be arrested if caught on the property. Then, the grandparents would be evicted for cause.
As for the lady in B offering to babysit, surely there is a clause in the lease that states the unit is for a primary residence only, not to be used as a business, etc. People can of course babysit every so often but not everyday and certainly not more than a day or two. Our lease stated that no person could "visit for 5 consecutive days" without applying and being admitted as a leaseholder if 18 years or older or occupant if under 18.
As to all the pot smoking, well, if it is illegal in your state, you can move to evict. I would definitely take photos and call the cops so I could get a report for documentation. *When I first started in Section 8 as a manager, it took me a year and a half to clean up the place. It was not easy because I was there walking the property and checking IDs of people on the property at 1, 2, 3, 4 in the morning and if they were not there for a justifiable reason, they had to leave (meaning they could not name a resident they were visiting, for example.) And if they had no ID, they had to leave. I know of NO management company out there that wants their employees to take such an active role. In fact, they will not support you, so I understand people cannot be there "policing the property" and so you do what you can do while onsite.
A last thought, when you say "over occupancy" are you referring to the rule that 1-2 people in a 1 bedroom apartment, no more than 4 in a 2-bedroom and up to 6 in a 3-bedroom? I have relaxed this standard depending on the size of the apartment space. Some of our 2-bedrooms have a HUGE bedroom and a regular 11 x 12 bedroom, so really, a couple with 3 kids could be comfortable in that really large 2 bedroom until a 3 bedroom becomes available. For the 1 bedroom, I think we all know couples who end up having a baby during their tenancy and we don't evict them when they bring the baby home.
I don't feel sorry for the granddaughter who is homeless and has no job. I feel sorry for her children. My obligation is to my property's residents and sometimes putting on my social worker hat comes in handy. During the meeting, you could direct the granddaughter to homeless shelter, to Goodwill, to drug and alcohol AA treatment, to a hospital, to apply for subsidized housing now so when she isn't doing drugs she might get an apartment. To the grandparents, CPS can help them become foster parents so in the event, the great grandchildren become wards of the court, they can take them. But you do not have to allow their mother onsite.
Sorry for the long answer.