Is it permissible for the husband to spend on his well-off married sister, and does the wife have the right to interfere and express her opinion regarding this spending?

It is obligatory for the husband to spend on his wife and children in a reasonable manner, and this obligation takes precedence over others. The wife is not obligated to spend from her own money; if she chooses to do so, it is a praiseworthy act of kindness. If the husband fulfills his obligatory spending, there is no harm in him spending from his money on his sister, and the wife has no right to object. However, she can advise him gently and wisely regarding the utility of his spending and balancing it with the interests of his family.