Is it only when he pays with he's sister? Ask him if anyone has been touching his family in school or church. Whether male or female. Lecture your children on sex education so that they wouldn't be abused
They are 2 possibilities. It's as a result of bladder spasms or purely behavioural. You might also want to check if he's on any medication recently. If this continues, I will advise you see a paediatrician
Medical Doctor