Hi keepingsecret - I'm so sorry to hear about your sister, a BC diagnosis is a shock for everyone on the family. I wish your Dsis all the very best for her treatment and recovery.
As others have said - cancer is a fairly common disease, so having family members with a diagnosis doesn't necessarily mean you are at risk, - some breast cancers just arise from spontaneous mutation, and are not related to genetics.
However I think your mum is sensible to advise getting checked out - it can certainly do no harm, and will set your mind at rest
Of course it is scary for you but even on the slim chance that they find something, BC is a very treatable disease caught early.
I hope we can support you,- there's always someone around with a sympathetic ear, and of course if your sister is a computer user, she'd be very welcome if she fancies a chat with people who have been through the same thing. 
Topsy - I hope that swelling goes down soon (when I had it, it felt as if I had a rolled up newspaper under my arm - quite odd) it is rather uncomfortable. I hope those tax credits get sorted out soon too.
love to everyone else. I went to yoga this morning - nearly fell asleep during relaxation, it was a meditation about lotus flowers - very soothing 