Relatively is carrying a lot of weight there.
Their 28-year-old son may not have much to do with the fact that they are having another kid, but that fully grown adult man is still a part of the family and should count for something.
There’s no need to just completely and totally toss him out of the equation.
Especially when you consider that there is a possibility, depending on the age of the girlfriend, that both of them could die before this child reaches the age of majority, in which case, he would likely need to step in to finish the job.
You’re saying that nobody else in the family matters even though they’ve already decided to keep the child.