Sorry to hear about your sister's problems. I am a three year survivor of rectal cancer. Her bowels may take a year or two to settle down, so she should just get to a point of acceptance about that. She will have a new normal, and it will not be as good as what she had before... but LIFE is better than the alternative, so one just has to accept the negative side effects as the new normal.
However, she does NOT have to stay home. She can work around the problems and everyone has different solutions. For example, there are special undergarments that she can wear, OR (my solution) is to wear a pad all the time to line my underwear. This gives me enough time to locate a restroom when out and about, and if there is a bit of leakage I simply change the pad, and keep clean with portable packages of baby wipes that I carry all the time. I never leave the house without a change of underwear... not that I need to use it very often, but it's good to be prepared. Another strategy is to identify where the restrooms are when out. I locate a restroom when I first enter an area, like the mall, a restaurant. A third strategy is NOT to eat right before going out if I'm not going to be near a restroom (like when I go kayaking or boating.) The most problematic activity is eating out (as I often have to use the restroom immediately after eating). So I eat small meals, and only foods that I know do not bother me. I avoid ground pepper and other very spicy foods. Other than that, I can eat most foods in small quantities.
Just to prove that this eating strategy works, I can tell you that I NEVER stay home, have a full and active life and that includes swimming, kayaking, biking, and walking several miles everyday. And I'm a retired 63 year old woman and love a new challenge so I'm trying new things everyday. I went on a cruise with my husband and two teen-age grandchildren less than four months after my surgery. I was still on chemo at the time (Xeloda, which has as a side effect loose bowels, or diarrhea.) The only accomodation I had to make was that I could only eat breakfast and dinner. If I ate lunch, I had problems with having to go to the bathroom and it would have interfered with our activities (snorkeling, kayaking, boating, BOB, snuba diving). So, I had a good breakfast, a few bites of yogurt for lunch (which is good for the intestines) and then had a leisurely dinner at the late serving at night. Try to have your sister develop her own strategies for living and not having to stay home. I NEVER stay home. Encourage your sister to go out... she'll feel better and it will make her quality of life better... do not give in to cancer and stay home. Stay positive!!