Depression is an entirely separate diagnosis - it is NOT necessary to DID. I'd call it "something else".

There is a support group called CODA for people with issues of co-dependence - which sounds VERY much like your friend (and the rest of her family). Co-dependence is an unhealthy family dynamic that can occur in any sort of dysfunctional family (addiction, physical disability, mental illness, otherwise troubled persons). And I'd suggest that the siblings are as much (if not more!) "at fault" as the mother. I don't BLAME anyone, but your friend is not living in in some vacuum of isolation - she does have other family who are making their own choices.

But in some ways, it sounds as if she's still living at home and being asked to help out around the house and with the family. Is she paying rent? If not, then it may not be that unreasonable for her mother to ask her to do chores in exchange for rent-free living. Baby-sitting for her sister? Well...It doesn't sound like this has ANYTHING to do with DID. It sounds like she's living at home and her family expects her to help out around the house.

Maybe she could start paying her mom some small amount of rent - that might help her feel more independent, and less pressured to do "everything" the family asks of her?