Survivor guilt. Watch "Ordinary People" again.

Thanks to your daughter you have a little guilt protection; you can do things on your terms, check first before you give, say "I'll see what I can do" and know you'll follow through with seeing what you can do because you are that kind of person; you may not be able to count on your sister but you can count on muffy, she'll do what needs doing. Logically, your sister is not going to starve in one evening? So, the next day is okay for checking things out, in the daylight, cabinets open, etc.

But start helping her when she's there. No helping on your turf; she needs help with her money/bills, she can make an appointment when you come to her place and work on her problems; no going out with friends while muffy is up, anxious all night! Offer to bring her food next time instead of money, see how she "reacts". Even offer to bring her some of your food, see if she can't develop either a little guilt of her own or have the decency to be embarrassed to pull stuff on you like that.

Her choices are hers. Treat her like a child if she insists on acting like one, give her children's choices, but on your terms, not hers.