My uncle visits me at home and browses through horoscope websites. I know that reading horoscopes constitutes disbelief in Allaah The Almighty, but he believes in these things. Should I prevent him from doing so, although I am sure he will be angry with me?

All perfect praise be to Allaah, the Lord of the worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.

The horoscopes that are published by some magazines and which claimed to tell  man's  fortune are considered soothsaying and this is prohibited. Therefore, a Muslim must neither read nor  believe in them.

Abu Hurayrah narrated on the authority of the Prophet, sallallaahu ‘alayhi wa sallam, that he said that anyone who  approaches a soothsayer or a sorcerer and believes him, has disbelieved in what was revealed to Muhammad, sallallaahu ‘alayhi wa sallam. [Ahmad and Al-Haakim]

 Believing in horoscopes constitutes disbelief in the revelation received by Muhammad, sallallaahu ‘alayhi wa sallam. Allaah The Almighty Says (what means): {Say, "None knows the Unseen in the heavens and the earth except Allah." And in no way are they aware whence they will be made to rise again.} [Quran 27:65]

Just reading horoscopes without believing in them is considered the same as approaching a soothsayer or a sorcerer. Some of the wives of the Prophet, sallallaahu ‘alayhi wa sallam, narrated that the Prophet, sallallaahu ‘alayhi wa sallam, said that if any Muslim consults a fortuneteller, his prayers will not be accepted for forty nights. [Muslim]

Therefore, you must advise him not to browse these sites. If he continues to do so,  you have to prevent him.  Anyone who seeks to please  Allaah The Almighty, even if he displeases people, Allaah The Almighty will be  pleased with him and will cause people to be pleased with him as well. [At-Tabaraani in Al-Kabeer]

Allaah Knows best.