Can’t give any medical knowledge on this just my experience.  In 07 my mum got breast cancer, giving all clear in 08 but it had spread to her brain and she died in 09 at 58.  At the time my two brothers and i asked the doctor if we were more likely to get cancer because our mum had and the doctor said it’s only potentially linked through family if you were to have the same cancer.  My dad died in 21 aged 73 from oesophageal cancer and my brother aged 38 has recently been giving a year to live with terminal lung and stomach cancer.  Maybe not linked but definitely not the best luck!  

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.

We asked about this when my wife got her BC diagnosis, because we also have a daughter. We got told the majority of cancers are random, meaning there's no family history to be had. The types of cancers, and the age of those having the cancers matter a lot. For example, breast cancer and ovarian cancers can be linked if there's 2 or more people within the same bloodline that have had the diagnosis. Even then, age matters a lot. Like a lot. Say your gran, who is 85 developed BC, it wouldn't be considered genetic due to age. So if gran had bc, and say, a young sibling developed BC, it would be put down to coincidence, which happens a lot. Doctors look for a pattern, say if your mum had had BC before the age of 50, and your sister developed ovarian or BC at the age of 34. That would be looked into. More so if there were more than two instances.

It is something that crosses your mind, because we have been there too, but it seems like the cancer in your family's case is just sheer bad luck. It's more likely 2 family members just get cancer through bad luck rather than genes. However, there are some genetic things that do make cancers more likely within a group of family members, but those are a minority. I think a lot of people just assume cancer is something that is mostly genetic related because that seems to be a myth that we have all heard and picked up on through the years. Not too dissimilar to how people still think you can catch a cold from being out in the rain. A lot of people still think this. The cold is a virus, not something you can catch from rain.