There is absolutely nothing unusual about making an inheritance conditional on something else. Inheritances are typically conditioned on the heir or recipient reaching a particular age or graduating from college.
What happens if a condition is questionable? Exist limits to the conditions that you can set?
There are certainly limitations, but they differ a little from state to state. Normally, you can not require anybody to do anything illegal or naturally unsafe. However, what about making an inheritance conditional upon the heir getting wed. There does not seem to be excessive questionable about that. Nevertheless, what if the person does not wish to get wed? What if he or she is homosexual and lives in a state that does not permit gay marriage.
The point is not to suggest that you should not leave conditional inheritances. You need to talk to an attorney about the limitations to the conditions you can set and the wisdom of setting conditions to manage somebody else’s behavior.