Is it permissible for a husband to give his sister money to change her car, even though she is well-off, while he constantly complains about debts and expenses and has not provided his wife with clothing since their marriage?

It is obligatory for a husband to spend on his wife and children in a reasonable manner, and clothing is part of the obligatory expenditure upon him. A man's spending on his wife and children takes precedence over his spending on others. If the husband spends on you and your children in a reasonable manner, then there is no blame upon him if he gifts his mother and siblings from his wealth whatever he wishes, and you do not have the right to object to him. However, there is no harm in discussing with him and advising him to balance his spending on his family with providing generously for his wife and children.