Please note that acellular pertussis vaccines used nowadays are not the same as the cellular ones used 30-40 years ago, modern ones last much less time. Also that the vaccines used 30+ years ago don't guarantee current immunity anyway partly because immunity wanes and partly because different strains are circulating now..
So while you don't seem to have caught pertussis from your MIL, so you really shouldn't worry here, it's really not true to say you are necessarily immune based on having been vaccinated as a child, or that the vaccine is just for your baby.
Everyone coming into contact with babies or the elderly (for whom pertussis is most dangerous) should be being vaccinated roughly every 5 or so years, which is how long the modern acellular vaccines last. To make sure noone gets it at all you'd want to vaccinate everyone every 5 years, but that's too expensive for most health sytems, which is why you still get epidemics of pertussis in 8 year olds whose immunity has waned but they aren't up to their 11 year old vaccine (at least in the country I'm in - i don't know the specific immunization schedule for pertussis in the UK).
There is good evidence that those epidemics are actually being driven by the 8 year olds' parents and grandparents who are no longer immune and don't know about it. This is partly because people (GPs, patients) don't recognize pertussis easily - many adults just have paroxysmal coughing fits with long breks in between coughing, and no whooping at all; and by the time they've had a cough for three months (it gets called the "hundred day cough" in China), the bacteria have long since disappeared (the cough stays on for other reasons than just infection), so if the doctor tests for the bacteria they're not there to find. Most GPs don't know that this doesn't rule out an initial infection with the pertussis bacterium!
BUT - don't worry - if you don't have a "hundred day cough" it's highly unlikely you've actually caught it from MIL!