You can structure finances in such a way that there is minimal exposure for your assets. However, to do this most effectively you should work with an experienced Family Law attorney and an experienced estate planning attorney.
The prenuptial agreement should be drafted by an attorney specifically experienced in drafting prenuptial agreements, because in addition to protecting you against the ex-wife, it will affect your financial arrangements with your husband. So with qualifications the prenuptial agreement should protect you as long as it is properly drafted and you and your husband do not co-mingle your assets.
Your fiancee's ex-wife does not have any claimable interest in your property through her ex-husband. Your income may be counted in deciding how much your husband should pay in support, but you should not have to pay simply because you marry this person.
You did not say if she has a judgment or claim against you personally. If that is the case then the prenuptial agreement may not protect you. That however, does not seem to be an issue in your facts.
Assuming for the moment that she does not have any claim against you, a prenuptial agreement will protect the assets you own and the growth or income from those assets. I would also recommend keeping separate bank accounts, investment portfolios, retirement funds etc..
Since you are not now married you have the opportunity to do some planning regarding how title is held on property. To take advantage of this work with an estate planning attorney.
Open a bank account for the community from which to make payments on community debts. Do not try to hide any of your fiancee's money in your separate bank account.
You may consider paying for your home from only your assets, if your home is separate property. Let your husband pay other bills such as groceries, utilities and insurance from either the community property account or from his separate property account.
But as I have pointed out you need to see an attorney to make sure everything is done right.
You're mis-informed, or at least incompletely informed, about the rules for calculation of child support in California. You should sit down with an experienced family law attorney to figure out whewther or not you need a pre-marital agreement, and even if you don't what other steps you should consider to protect your finances.
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