Some thoughts: (I've been a family law lawyer for 31 years, married (WITH A PRENUP) to another family law lawyer for 20 and law partners with my wife for 12):
A. I believe that if you and your fiance, or you, your fiance, and his family, are going to have issues about finances, and expectations, it's better to find out BEFORE you get married, than afterwards. While talking to your fiance openly and seriously about your and his expectations may make the prospect of marriage a little less romantic, it's the best insurance against divorce you can get.
B. At least under California law, if your fiance inherits property from his family, or receives it by gift, that's his separate property, even without any premarital agreement, as long as he keeps it identifiable and separate.
C. You should not even think about signing a pre-marital agreement without consulting an attorney with substantial family law experience. Your fiance and his family will WANT you to do so, because otherwise the agreement will be much more difficult to enforce. If they really want you to do so, they should be willing to pay for you to consult with an attorney YOU CHOOSE, to review the agreement.
D. Premartital agreements are negotiable. If you want some terms other than those which your fiance's family proposes, then you should propose them.
E. "Cheating clauses" are almost certainly not enforceable under California law.
Congratulations, and good luck.
Prenuptial agreements are often more about negotiation than the law, however it would be in your best interest to have an attorney review the proposed instrument and fully explain your rights to you; you may be giving up more than just a right to his family's money. California already has measures to protect your fiance's inheritance should you two get married without a prenup so my inclination is that the agreement covers more than just those assets.
You can test the waters with your fiance and future in laws by asking if they would be willing to cover the expense required to have an attorney (of your choosing) review the agreement. Questions like that often give incite into how one-sided they feel the agreement may be.
If you're asking whose money the two of you would use during your marriage, unless that is specifically covered in the prenup, marital expenses would be covered by your community property and joint assets.
Ultimately if you don't feel comfortable with something in the agreement and he isn't willing to at least discuss alternative provisions that may be the answer you're looking for.
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