Complex sentence: According to the authentic hadith of the two weighty things, the words of the household of the prophet completes the Prophetic religious sciences and his household are inerrant in explaining teachings of Islam.
Split into simpler sentences: According to the authentic hadith of the two weighty things, the words of the household of the prophet completes the Prophetic religious sciences, and hence are authoritative. Also, his household are inerrant in explaining teachings of Islam.