All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
Being kind to widows and orphans and donating to them from Zakat money when they are rich is not permissible because Zakat cannot be paid to a rich person. The Prophet said: "Zakat is not permissible for a rich person." [Abu Daawood] However, there is no harm in giving them optional charity but it is better if it is given to those who are more in need of it. So it is more appropriate to be kind to widows and orphans who are more in need of help.
As regards the issue of your cousin, he is your agent to run the house for you. An agent is not permitted to act as he wishes except within the limits to which he is entrusted. If the matter is as you mentioned that he let his sister live in the house without your permission then he was wrong. He and his sister have to be advised about their mistake. Indeed you have the right to evict his sister from this house or that she pays rent. If she does not move away or pay the rent, then it is permissible for you to take the matter to an Islamic centre so that they would look into it and take the necessary measures. If this is not possible, then there is no harm in taking the matter to any authoritative person who can force them to fulfil your right.
It should be noted that one should be wise in tackling such a situation in order to keep the ties of kinship, and take the easiest method in solving this problem. Besides, it should be noted that when an orphan reaches the age of puberty, he is no longer an orphan, because an orphan is the one whose parents (or one of them) died while he has not yet reached the age of puberty.
Allaah knows best.