I have no medical knowledge and have only picked up general information about cancers that have been in my family. Bearing in mind that 2 out of 3 people will get cancer at some time in their life the odds aren't good, but it doesn't necessarily mean that all types of cancers run in families. My paternal gran died from lung cancer (passive smoking from my grandad), my paternal aunt died from ovarian cancer and her cousin died from the same. My mum died from ovarian cancer so I have it on both sides of the family but it only raises my chances of getting it slightly. However, I have had melanoma for many years and am the only person in my family to have it. My only sibling and his two children have had precancerous moles removed though. A percentage of Melanoma is caused by genetics, not sun exposure, so can run in families - however mine was caused by sun & sunbed use in my 20s so the chances of my family getting it would not be down to genetics. This shows how it's a lottery when it comes to cancer. 

Angie (stage 3 melanoma patient since 2009)

Forgot to mention, for melanoma (and I assume other cancers that can be genetic) there have to be at least 2 or 3 close members of the family that have that particular cancer for them to be able to access genetic testing. If your mil is the only member of your husband's family to have breast cancer and she doesn't carry  any of the genes, the chance of any member of her family getting breast cancer is the same as for any other person.

Forgot to mention, for melanoma (and I assume other cancers that can be genetic) there have to be at least 2 or 3 close members of the family that have that particular cancer for them to be able to access genetic testing. If your mil is the only member of your husband's family to have breast cancer and she doesn't carry  any of the genes, the chance of any member of her family getting breast cancer is the same as for any other person.