Is it permissible for a brother to force his university-aged sister not to go to her university alone, knowing that her guardian (the father) does not object, and that the sister's refusal may cause her psychological and physical harm, and is the brother sinful if he lets her go alone?

If the sister goes to university using public transportation without being in seclusion with the driver, and no fitna (temptation/strife) results from it, then there is no harm. Whoever sees a munkar (a reprehensible act) should advise and guide gently and wisely. If the person does not desist, then one should inform someone who has authority over them. The brother does not have the right to use force with his sister as long as her guardian is present. He can inform the father or advise her in the best possible way, for wisdom and gentleness bring obedience. Allah Almighty said: "It is by the great mercy of Allah that you were lenient with them. Had you been harsh and hard-hearted, they would have surely dispersed from around you." (Aal ‘Imran: 159), and "Invite to the way of your Lord with wisdom and good instruction." (An-Nahl: 125). The father is responsible for his daughter and for preventing her from munkar. Her merely going out in public transportation, without a long journey, is not a munkar in itself, but she must adhere to the Sharia (definition) guidelines.