to make Petunia desperate...

You decide to remove Dudley from the family completely by turning him in for murdering his own father. Petunia would then eventually be desperate for sex and thus slowly breaking and corrupting her.

You plant the evidence and Confund the lead investigator and made him to recheck for evidence. The police take Dudley away and sentence him to life imprisonment for the murder of his father. Petunia was shocked to hear that and that left her heartbroken. She then confessed on Dudley's attempt on her which lead to Dudley's life sentence.

Regardless of such dire events, Petunia's dominant nature was evident as for the next few days she tried making you lick the floor clean from her spilt coffee, and occasionally tried peeking in your bath sessions.

You, however, did not let that slide and jacked off at every time she would take a peak.