I'm sorry to hear about your Nan and Auntie. I assume you're talking about the BRCA1 and BRCA2 genes? Before I say anything, there's a very good website about them here.
First of all, there's no guarantee that your mum, or you, have inherited mutant BRCA1/2 genes (the type of the gene that increases cancer risk). So just hang tight, and wait for the test results from your mum.
Quick Overview of BRCA The BRCA genes are genes that produce tumor suppressor proteins - these are just proteins that control cell growth. People with mutated BRCA1/2 genes have faulty versions of these proteins. These specific genes (BRCA1/2) are responsible for DNA repair. When the proteins don't function there's an increased chance that the cell's DNA will gain mutations (changes in the DNA) which can cause cancer.
It's a bit more complex than that, but that's the simplified version. If you're interested Wikipedia has some more science-y info!
Positive BRCA test If you test positive for a harmful mutant of the BRCA1/2 gene this means you have an increased risk of breast and ovarian cancer. It should be noted, that even with inherited faulty BRCA1/2 genes you are not guaranteed to get cancer. The increased risk depends on whether you have a mutant BRCA1, BRCA2 or both. The exact percentages can be found here.
But it definitely does not spell impending doom!
There are preventative measures which can be taken, see here.
Negative BRCA test A negative test result is a bit more complicated. It depends on your family history and the test results of other relatives. Have a look here.