As others have mentioned, the decision to test is ultimately up to you. I would suggest you seek the advice of a certified genetics counselor to guide you, however, and not just have your OB-GYN draw your blood and send off for testing. As Judy said, you need to look at a lot of other things such as family history. The good news in all of this is that since you know your sister's specific mutation(s), that is what they will test for rather than looking across all the spectrum of the BRCA1 and BRCA2 mutations. That will be less expensive (only a few hundred dollars) than the full panel, which is $3-$4,000 right now. Most insurance plans cover the testing, if deemed "medically necessary".
If you are not a mutation carrier, your risk of developing breast cancer is the same as the rest of the population - your risk is not elevated.
If you do have the same mutation(s) as your sister, then you have other options to consider - increased surveillance is definitely one of them. However, it does NOT mean you will get cancer - only that you have an increased risk.
I suggest you contact a genetics counselor in your area. If you don't know of one, you can look one up through a variety of resources, one of which is Facing Our Risk of Cancer Empowered,
www.facingourrisk.org . It's a non-profit org dedicated to those at risk of hereditary breast and ovarian cancers. There are also links to other resources and support groups there where you can post questions, too.
I am a BRCA1 mutation carrier, and did not know my status until after my sister passed away (at age 34) from breast cancer and my mom (at age 69) from ovarian cancer, and I myself had been diagnosed with breast cancer. I wanted to know "why". I have since learned a lot.
I wish you only the best in your decision-making.
Hugs,
Nan/PinkLady