Just one night before, Ya’qub (PBUH) said to them:

  وَأَخَافُ أَن يَأْكُلَهُ الذِّئْبُ وَأَنتُمْ عَنْهُ غَافِلُونَ 

(and I fear that a wolf would eat him while you are of him unaware.) (Yusuf: 13)

So when they came to their father, they said: ‘A wolf devoured him.’ The lie was very obvious.

(… وَمَا أَنتَ بِمُؤْمِنٍ لَّنَا…) (… But you would not believe us…): Meaning, you do not believe us.

(...  وَلَوْ كُنَّا صَادِقِينَ ) (…  even if we were truthful):  The brothers’ excuse is reminiscent of the aphorism which declares that ‘the suspect all but incriminates himself’. They knew for certain that their father would not believe them, so they brought evidence.