So sorry about your sister.
My husband did two cycles of Avastin and then stopped because it did not work for him. However, it was worth a try and it was his final try as far as treatment was concerned.
If Avastin is effective, I believe that results are almost immediate. It's not something that you have to wait for weeks to figure out if it is working.
So I would vote to try it as your sister's quality of life is so poor. To convince your mother and other sister, perhaps telling them that the results will probably come quickly and if it doesn't work, she just stops at one treatment. I think most of the side effects happen with extended use.
My husband also felt very ill on temodar. Not quite as bad as your sister, but he was miserable on it. He had one good week a month.
I hate how there are so few good treatments options for GBM. Avastin is one of the few meds that gives some people some good quality time.
God bless. Hope you figure out something out, Sally