First, I am very sorry to hear of your sisters diagnosis, and at such a horrible time, that being during a pregnancy, not that there is ever a good time for this to happen to anyone. I personally don't believe in most of the life expectancy prognoses that the doctors give at initial diagnosis. Everyone is different and responds differently to treatments. Your sister had nearly 100% of the tumor removed surgically, and that is an important first step. Seeing as she is pregnant, I am assuming she is relatively young and possibly in otherwise good health. That all bodes well for her responding to treatments and keeping her strength up.
Second, I think you probably mean 6 weeks of radiation instead of 6 months. The "standard of care" for this kind of cancer is usually 6 weeks of radiation therapy with Temodar (chemo pill) every day for that 6 weeks. Following that most patients get an MRI and then begin what is known as 5/23 therapy which is Temodar for 5 days in a row, then 23 days off. Generally this continues for 12-18 months beyond radiation.
What makes this so difficult, in my limited understanding, is that while your sister can take radiation treatments while pregnant, I do not believe she can get chemotherapy because it is too toxic to the child she carries. There was another similar story on here recently, perhaps you can find it on some recent pages and can network with that family, perhaps get some insight. I believe their options were radiation only or do a "medically necessary abortion" to continue on with both radiation and chemotherapy. Again everyone's situation is different, and I would hope that your sister's doctors are giving you all the best information.
I hope you will find support and some answers here, I certainly have since my wife's diagnosis just over a year ago. She was just 32 at the time of her diagnosis, we were just married and contemplating things like starting a family, etc. No one is prepared for this when it falls in their lap, the best you can do is cope and seek information. People do beat this disease, and our neuro-oncologist has used the phrase "effectively in remission" to describe my wife's progress. You can't know what will happen until it happens, good or bad. You just have to keep on fighting. All the best to you and your sister.
-Brandon.