The huge colon sometimes becomes so large that the diaphragm may be pushed up. These are all thought to be due to weakened smooth muscles in the digestive tract, although to find the exact cause we need more studies. Diarrhea is not described in the literature well. However, I have seen patients with myotonic dystrophy with diarrhea and constipation like you and your sister and nephew. However, the fact that only affected members in your family have these symptoms makes me think that this may be related to myotonic dystrophy.
Treatments are conservative.
For constipation, Metamucil, laxatives and enema are usually used, depending on the severity. I would avoid spicy foods to prevent abdominal cramps. Kaopectate is a reasonable solution for diarrhea, but as you mentioned, it may cause constipation. Present the above information to your gastroenteroloist and discuss the treatment right for you. The first time I had a paralyzed bowel.