What is the ruling on buying a mobile phone for a sister without the father's knowledge, and does sin result from that?

If you are mature and have control over your money, and your sister uses a mobile phone for permissible purposes, then it is permissible for you to buy her a mobile phone without your father's knowledge. However, if your father objects to your sister owning a mobile phone out of fear that she might use it for impermissible things, then it is not permissible for you to assist her in sin.