This question is too complex for a simple answer. First, you need a Good Family Law attorney to advise you and you need it now. Secondly, you need to state if your business is incorporated or not. Third, how are you compensated from the business?
Generally, a prenuptial agreement is dependent on the State in which you live. Each state has their own rules about these things. I practice in a community property state and our rules are very different than a common law state. Illinois is a common law state, as I recall. In Washington, you cannot force someone to enter into a prenuptial agreement. The closer to the wedding you are the less likely a prenuptial would be upheld. You are on the edge at 5 weeks. If your fiance won't sign such an agreement then you must choose marriage or no marriage.
Go to a lawyer right now and get this ironed out before it is too late.