That's sad. The best bet would be to try to communicate more in a very non judgemental way with his mother. Is she concerned about calcium? Yogurt isn't as constipating as other dairy. Vit C supplement would help soften the stool (also helps calcium absorption). I talk with my ds about what foods make stools hard and what ones make it soft. Your sister could get a ring seat for the toilet (I have a nicely contoured Bjorn one, or she could get a cheaper padded one) to make it more comfortable.