I don't think you are doing him any favours by letting him get away with it. He needs to take responsibility for his own finances for the sake of his own future happiness.
I know it is hard when it involves family, but next time he comes, tell him that he owes that money and must start paying it back, preferably by a regular direct bank transfer credit, even if it is only at 5 or 10 quid a week. Explain that it is hurtful for his sister to see how getting a dog is more important than her.
I know there is a risk here of a rift: I've been through that and I know how it hurts, but I reckon he relies on you all too much to stay away.
Whatever does happen, one thing you can certainly do is ensure he gets no more money. As mentioned before, if he uses the children/no food blackmail. just buy the food. If he uses the electricity cut off excuse, ask for the bill, ring them up and arrange for him to pay it off in instalments if possible, otherwise refuse to pay it unless he sets up a direct debit to pay you back.
Good luck!!!
Angela Rayner lashes out and calls Sunak “pint sized loser”.