Whether the baby was planned or not is not really the point.
If you are in a sexual relationship, and of childbearing age, then a baby is always a possibility. This is an issue that should have been talked about a long time ago, rather than swept under the carpet.
You cannot ban your stepchildren from their home, or from seeing their dad.
It is also worth noting that an unvaccinated child is not a walking germ factory (well, not anymore than any other child!). I have 2 unvaccinated children, and one partially vaccinated child. Of course I worried each time I had a newborn. But dd1 not having completed her jabs did not mean that she would automatically infect any subsequent babies with every illness going.
and dd2 being unvaccinated did not mean that she automatically infected ds when he was born.
the vaccination issue is not black and white. no one has the right to issue ultimatums over vaccination.
out of interest, OP, do you know your own vaccination status? or your DP's? if your DP is not immune, do you plan to keep him away?
fwiw, the closest my children have come to whooping cough was when my uncle had it last year. he did not initially realise it was whooping cough because, as said before, he didn't whoop. it was only when it lingered that his gp suggested whooping cough.
it is not just childrne you need to worry about.