I assume he has encopresis which my son also has. Your sister needs to get him into the GI group at Children's Mercy. It may be as long as a 2 month wait so she shouldn't delay getting him in. Also, if she thinks he is getting better she should still go to the appointment. Here are my suggestions.
1. Clean him out over a 2-3 day period. You do this by giving him large doses of Miralax so that his bowel gets completely cleaned out. Probably 4 capfuls daily for 2-3 days. Put it in gatorade or juice. His poop should be very watery by the last day to make sure his bowels are completely clean. My son calls it his pee poop.
2. After the cleanout, he should take a capful of miralax daily to ensure he is pooping 2-3 times daily of soft poop. I suggest that they schedule time on the toilet 20-30 minutes after mealtimes. Don't have him sit for more than 5 minutes. If he isn't sucessful, have him try again 45 minutes later.
3. Try a rewards with him. For a successful BM, maybe he could get a piece of candy or something. Also, use a sticker chart. Figure out something he really wants and have him work towards it. I would start with 5-7 days of clean pants to get a reward. Don't make it consecutive days as this will frustrate him. You can then make the time period longer with a bigger prize.
She needs to get on top of this now as it will get harder once he gets older. We are now seeing a psychologist for my son who is almost 7 and he says if we are fixed by 8 we will be okay.
As far as the underwear goes, I disagree with some of the other posters about pullups or diapers. I know it gets old cleaning out the pants and we too have thrown alot of underwear away. We eventually started taken the underwear out to the yard and using the jet spray on the hose to clean them out. This worked very well.
I may be rambling on but I know how your sister feels. We have been dealing with this for over a year now. If you or your sister would like to talk to me about it, please let me know. I feel like I am the poop queen and I know it is hard to get the doctor's time and attention.
I forgot to mention. You will need to titrate the Miralax up and down until you get the right dosage for him to have 2-3 soft bowel movements per day. We had to add in some benefiber at the end to make the stools firm enough for him to feel. Also, if he is having accidents once you have cleaned him out, he is not getting enough miralax. What happens is the watery stool gets around the solid stool that is in the bowel. We saw Kathy Christensen who is a Nurse practitioner in the GI clinic at Children's mercy. They have several NPs so you might get someone else but please get on their calendar.