If he has been to see a doctor I am assuming encopresis has been ruled out. If it is not encopresis or some other physical problem, my bet would be it is probably emotional. A nephew of mine had the same problem at around 8 years of age (this was due to abandonment by his mother). Your SIL getting angry about it is only going to compound the problem, sounds like he and your daughter need to get on the same page before trying to remedy the little guy's issue. Also talking about the problem in his hearing or making a big deal of it is certainly not going to help either. Perhaps a regular, peaceful and fibre rich routine may be the answer....just dealing with the "accidents" quietly and efficiently and then "moving on" to the next activity or whatever. Taking along wipes and spare underwear, getting him to tell a grownup when the accident happens. Maybe pre-school needs to be put on hold, 'til there is a significant improvement, no point in putting him under more stress unnecessarily. If your SIL had the same problem I would have thought he would have been more understanding....perhaps some family counselling might be in order, rather than just focussing on the child.