Is eating fruit and food bought by a sister whose husband has died and who has children and is an orphan considered consuming the property of orphans, and is it therefore unlawful?

There is no objection to eating the food of family and sister, with their good will, even if it is mixed with the food of their orphans, as long as this is necessitated by need and is customary. The righteous predecessors used to do that, and this is supported by the words of Allah the Almighty: "And they ask you concerning orphans. Say, 'Improvement for them is best. And if you mix with them, they are your brothers. And Allah knows the corrupter from the reformer. And if Allah had willed, He could have put you in difficulty. Indeed, Allah is Exalted in Might and Wise.'"