Is it permissible to take from the charity funds given to a widow to pay off the debt for her house furniture that was prepared for her without her knowledge, for fear that this might be misunderstood as greed on her part?

It is not permissible to take the sister's money without her knowledge and consent. If you want the price of the furniture, you must inform her and obtain her approval. Fearing her anger is not a justification for taking her money without her consent. You either donate the furniture to her or demand its price from her.