Wow, some people are harsh!
What does your husband think, OP? It seems to me very clear that there is no one right answer here, and the main thing would be to find a solution that will keep everybody happy/not lead to resentment. I doubt anyone online can know what that will be.
ETA, for example, this could be an evil stepmother scenario. Or it could be that the families, having lived together for 10 years, are fully blended and the kids are treated and loved equally. We can’t know. But I would also add that this is not even as much a financial decision as an emotional one. Will someone feel less loved because they got less money? Unless the parents are able to talk about this and come up with a solution that works for all, I would suggest working with a therapist/counselor so the emotional aspects don’t end up tearing the family apart. (And of course, can’t tell if that’s even a remote possibility).