Hi GGgg,
Welcome to the Forum and glad that you have asked for advice, you must be really really worried about your sister, it really sounds as if she needs help and needs it very soon.
Her children have to come first, and I would be very worried about them as I am sure you are. Does the friend go round and check on them and your sister frequently? How old are they? They must be very sad with losing their Dad as well.
I really don’t know what I would do in your situation, but I would have to think of the children and if you think they are not being looked after properly, then you could involve Social Services, it might bring your sister to her senses.
There is also the Icarus Trust who could give you advice as well.
Keep in touch through the Forum, it is a very useful place to come and get really good advice.
Dx