The word "inheritance" is a red herring here.
Basically, the in laws had some money and gifted it to their son.
If the tried to ring fence if then it would not be a gift, it would be a gift with reservation, which brings its own problems.
So the next question is, having received the gift, could your husband ring fence it somehow? The answer is no, not if he has used if to purchase what is then the marital home. There is potential for spouses to ring fence assets in some circumstances, but only where they are not intermingled with marital assets not relied upon by the spouse, and even that depends on the length of the marriage and whether there are kids, his the kids'needs would be met etc. And it's all challengeable in a divorce.
The other option would be for the in laws to "buy into" your house. Then they would own that portion. They would always own that portion, but that has tax implications of course.
Spouses can own different proportions of a property than 50/50 but that doesn't really mean anything when you are married and it's a marital asset, which means it's owned equally anyway. Divorce laws impact on property ownership.