Does money given to one's sister or wife count as charity (sadaqah)? And is its reward equivalent to the reward of giving charity to the poor, even if they are neither poor nor rich, are content with food and housing provided by their husbands, and the charity is given to them for clothing, secondary life expenses, or sometimes essential needs?

Your spending on your wife and family is among the greatest acts of charity. As for what you give to your sister and your wife beyond the obligatory maintenance, if you intend it as charity, then it is charity. It is permissible to give voluntary charity to those mentioned, even if they are wealthy, but giving it to the needy among the poor is more deserving and brings a greater reward.