What an awful dilema for you - you must already be run off your feet with your own family.
I would personally seek professional help, find out all the options, Social Services actually want to keep families together.
You could call Social Services and speak to them in confidence you do not have to give you name or your sisters, but they will tell you everything you need to know.
Far better that you do it, because it won't be long before someone else does report her!
I have a friend who lost her husband last year and she had 5 children, whilst she was receiving help , her children went into a local short term foster home, spending weekends with their grandma - after 5 months all was well and the children are now back with their mother.
Best of luck.