Shame Marijke!!
The washing of underwear is still fresh in my mind....I have the deepest sympathy!
Not fun.
If it is a longstanding problem like you have said I wouldn't personally start major home treatment myself but maybe giving a very mild laxative can help. I know my Urologist suggested Lacson. Since this seems to be a medical problem I would suggest your sister take him to a doctor when she comes back. I can imagine the accidents are not very good for his self-esteem. Shame, poor boy!
I wouldn't reward him for going because if it is truly encropesis he can't control it therefore it is cruel in my opinion to treat him as if he can help it. Perhaps you could ask him to just try to go to toilet after a meal?
Perhaps the change in routine with his parents away worsened the situation emotionally as well.
I thought this article summed it pretty well:
kidshealth.org/parent/general/sick/encopresis.html#
Good luck...sounds like a difficult time ahead for you...