Should she stop her prayer because her family does not know that she is Muslim?
Question

                I am a new muslim and this is my 2nd ramadan fasting. My family is still disbeleivant and I pray and fast without their knowledge. At home, I pray only fajr and Isha prayers, and other prayers I pray outside ( i.e. at school, at muslim center or at friend's house). Whenever I pray at home I lock my bedroom door and pray. My sister also shares bedroom with me. This is what happened when I was praying Isha prayer at home. I locked the door before I started praying but somehow it didn't lock. I prayed first rakaa and I got up from sujud and reciting alfatiha and suddenly my sister opened the door. i got scared and i did not finish praying isha at that time i let her in. and after few hours i prayed isha again. i just want to know, what is the ruling on not praying while you intended to pray 4 rakaa and what should I have done.
Praise be to Allah.
    Firstly: 

    We congratulate you for the blessing of Islam, and we ask 
    Allaah to make you steadfast and to help you and strengthen your faith. And 
    we ask Him to guide all your family and your siblings, so that you will be a 
    means of their success. 

    Secondly: 

    If you are afraid to declare your Islam and fear that some 
    harm may be befall you, or if you want to delay telling people until you 
    have learned more about Islam and are able to call your family to Islam, 
    there is nothing wrong with you doing that. There is also nothing wrong with 
    your stopping the prayer if you are afraid that someone will see you. 

    The basic principle is that when the Muslim has started an 
    obligatory prayer, he is obliged to complete it, and it is haraam for him to 
    stop it, unless he has an excuse. 

    The fuqaha’ have listed some excuses which make it 
    permissible to stop the prayer. These include: 

    -Fear of being harmed by an 
    enemy or a wild animal.

    -Fear for one's wealth or 
    property, such as if a person is praying and someone comes who wants to take 
    his property, or he fears that his mount will run away.

    -Stopping the prayer in order to 
    save someone from a fire or from drowning, or fear that a blind man may fall 
    into a well.

    See Radd al-Muhtaar, 1/654; al-Mabsoot, 3/2; 
    Kashshaaf al-Qinaa’, 1/380. 
If you fear harm to 
    yourself if any of your family see you praying and find out that you are a 
    Muslim, there is no sin on you if you stop praying at that time, then pray 
    after the danger has passed, which is what you did.

     Thirdly: 

    You should strive to learn the rulings of Islam and keep 
    company with good believing women, who will help you to obey Allaah. Think 
    about the best way to call your family to Islam and strive to do that, for 
    they are the most entitled of all people to your good treatment and da’wah. 

    May Allaah help us and you to do that which He loves and 
    which pleases Him. 

    And Allaah knows best.