Julesage - sadly I have some recent experience with this question because one of my best friends died of brain cancer last summer. A few years before that a friend of the family battled lymphona and beat it.
If your sister has any reservations about chemo she should get a second opinion, but if both doctors feel it is important to help stave off a re-occurence of the cancer, then I think it's important to listen to them. Also, I'm pretty sure that a detox while doing chemo would be extremely difficult and possibly dangerous on the body. For one thing, the chemo drugs need to be in the body to do their job (in short kill of the bad cells). Plus, the body will have a reaction to the chemo and to suddenly throw it into something as extreme as a detox could make it a lot harder to recover and return to homeostasis.
That said, my friend who died did have a naturopath who worked in conjuction with her oncologist and they were able to work on ways to help keep the chemo side effects down and keep her more comfortable. But like with B.R.'s experience, brain cancer is pretty impossible to beat and she fought the good fight and lost. But I don't think it's because of failed treatments or because naturopathy or chemo is bad, I think it's because brain cancer is just a nasty close to impossible cancer to recover from.
For my two friends the most important component was that the naturopath and Western medicine doctors knew what each other were doing and made sure not to prescribe things that would be contraindicated.
As far as chemo goes, I took a chemotherapy drug the first year after I was diagnosed with rheumatoid arthritis. When I took the pill form it made me sick to my stomach so I switched to the injectable form and didn't have any side effects. Some friends were horrified - "why would you willingly put poison in your body???" Well, because the poison got me from the place where I couldn't make a fist or pick up a pencil or walk farther than a few feet with extreme pain, to being able to go jogging, start knitting again and ride horses. Chemotherapy is not all bad, especially if it can save someone's life.
Anyway, I'm sorry you've had to go through so much with your family being sick! That must've been really awful. I'm glad your sister is going to be ok!