As-salamu alaykum,
It must have been a shock for you to hear all of this and it may take some time for you to adjust to the news that your sister is in fact not your biological sister. While there seems to be an exaggerated sense of protection from your father to your sister, you may never know the real circumstances of her adoption. Perhaps your father is trying to protect her from some very sad news or devastating information that may hurt her later on but we do not know. Perhaps he knew her mother and in fact perhaps he knows who is the father. However, for whatever reasons, he fears disclosure, only Allah knows. There are Islamic laws pertaining to this and insha’Allah you father will be able to adhere to them one day as he realizes that as a Muslim, it is the right thing to do. Until then, I would kindly suggest that you listen to your father concerning his requests, treat your sister like you would any sister and make dua that your dad does the right thing in regards to the true identity of his adopted daughter. While I am not an Islamic scholar, I do know that adopted children do have rights and insha’Allah, Allah will touch your father’s heart, or relieve his feelings of anxiety or whatever it is that is troubling him about this-and free his mind to do the right thing. Until then, as you are still a child in his home, and really, this is a matter for your parents to resolve, especially your dad, try to exert mercy, forgiveness and patience. It is not your adopted sister’s fault, but rather a bad decision by your father to not disclose her adoption for whatever reason, so please sister, don’t take it out on her. Insha’Allah, Allah SWT will reveal all in a way that does not harm her or others involved. “…Nor has He made your adopted sons your (biological) sons. Such is (only) your (manner of) speech by your mouths. But Allah tells (you) the Truth, and He shows the (right) Way. Call them by (the names of) their fathers; that is juster in the sight of Allah. But if you know not their father’s (names, call them) your brothers in faith, or your trustees. But there is no blame on you if you make a mistake therein. (What counts is) the intention of your hearts. And Allah is Oft-Returning, Most Merciful.” (Qur’an 33:4-5).
You are in our prayers sister, please let us now how you are doing.