The current recommendation is that anyone who is around high risk people (elderly, newborns, immunocompromised etc) have the vaccination every 5 years. Pregnant mothers are now being told to have the vaccination in the 3rd trimester of every pregnancy, regardless of how long it's been since last vaccination, to allow some protection to the unborn baby.
I know the old recommendation was that as long as you had a booster every 10 years you were covered, but with the higher incidence in whooping cough of late I *think* the general recommendation is becoming every 5 years now for the general population. I could be totally wrong of course. I have only ever worked with high risk groups so have been vaccinated myself multiple times.
I would just send out a general text/Facebook announcement and say that anyone who would like to visit within the first 6-8 weeks should be vaccinated. I would sit your step kids down and explain to them that you would like them to have the vaccine and why you would like them to have the vaccine. I think it's reasonable to ask them yourself, and as PPs have stated, they are both old enough to consent to having the vaccination without their mothers permission. Obviously your DH would need to be included in the conversation as well so he doesn't think you've gone behind his back. If you try not to get emotive/upset about it you'll probably have a better chance at getting both your SSs and DH on board. I definitely think it's worthwhile batting for the unborn child though, it's a fight worth fighting!