Molly, Its important that your sister deal with this problem as soon as possible. I went from being uncomfortable and rare painful stools to not going at all and becoming impacted. I was in such pain and so impacted that the doctors and nurses weren't able to manually stimulate things. I was having surgery to put in the feeding tube and they took care of the problem while I was under anesthesia. To prevent future problems my doctor suggested milk of magnesia on a regular basis - I used as small an amount as I could. The problem decreased as I eventually got back to "real" food. As you have already read this is not an uncommon problem.
ilene