What is the effective treatment for the obsessive-compulsive disorder that my sister suffers from, which makes her exaggerate in purification due to touching dogs or touching impermissible things, knowing that previous advice has not been beneficial?

Waswas (obsessive whispering/thoughts) is from the work of Satan. The cure for waswas is to ignore it, not pay attention to it, not surrender to it, and completely turn away from it. When it is not given attention, it disappears. Whoever listens to it will be increasingly afflicted by it until it drives them to madness. One should believe that waswas is a satanic thought and fight against it.

Among the things that help in expelling waswas are: remembering Allah the Almighty, seeking refuge with Allah from Satan, reading the Qur'an, especially the two Mu'awwidhatayn (Surahs Al-Falaq and An-Nas), supplicating and seeking refuge with Allah, increasing acts of obedience, and staying away from sins and transgressions.