Sanphit Kopbo, 22, was wanted under a warrant for allegedly raping the 12-year-old girl, police said, adding that the arrested pair had also tied up the grandmother before stealing B300 in cash and fleeing the family home in Sadao district on a motorbike.
Both were arrested as they were heading to Muang district.
During questioning, Sanphit denied any wrongdoing, saying his girlfriend sexually assaulted her own sister using a cucumber, police said.
The man claimed he was sleeping in a separate room at the time of the attack.
Sanphit also claimed his girlfriend attacked her sister because she thought the younger girl was the reason why she was always getting scolded.
Police were not convinced by the story, given the fact he has a criminal record.
Besides drug offences, Sanphit was jailed for raping a minor in 2016.
After being released last year, he had just moved in with the older sister at the home she shared with her 73-year-old grandmother and younger sister, police said.
During a search at the house, messages were found on the elder sister’s bedroom wall saying how madly in love she was with Sanphit, according to police.
The alleged rape victim is now being cared for by her father and paternal grandmother at a relative’s home in tambon Phang La.
The father later dismissed Sanphit’s claim the older girl had sexually assaulted her own sister.
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