Is it permissible for the questioner's sister to enter her house after she swore not to enter it and told her, "It is forbidden for you to enter my house"?

It is advisable to refrain from excessive swearing by Allah, and it is forbidden to swear by other than Allah, as the Prophet, peace and blessings be upon him, said: "Whoever takes an oath should swear by Allah or remain silent."

If one swears by Allah and then sees something better than that oath, he must expiate his oath. The expiation for an oath is feeding ten poor persons or clothing them, or fasting three days for those who cannot find the means.

As for swearing by what is forbidden (such as by saying "it is forbidden for me" or "it is unlawful for me" to do something), the preponderant opinion is that it is considered dhuhur (a pre-Islamic form of divorce), and the expiation for dhuhur becomes obligatory if he allows his sister to enter his house.