Is it permissible, in Islamic law, to use a paid educational website via a secret code obtained from another person who paid the fees, without paying the entry fees?

It is not permissible for you to enter the site using your sister's name without paying the prescribed entry fees, as this involves deception and fraud. The Prophet, peace and blessings be upon him, said: "Whoever deceives us is not one of us." Therefore, fear Allah concerning people's money and rights. The Prophet, peace and blessings be upon him, said: "Indeed, I hope to meet my Lord and none of you demand from me an injustice concerning blood or wealth."