How should one deal with a sister who does not wear the hijab after repeated advice, and is it permissible to boycott her for this reason, or to stop financial aid for fear of it being used for adornment?

Your duty is to advise your wife and invite her kindly to wear the hijab, explaining its ruling to her, as well as the danger of displaying her adornments (tabarruj), and the beauty of chastity and modesty. You should also encourage her to keep good company. If shunning her or withholding her allowance will make her wear the hijab, then do so; otherwise, do not.