Is there any way I can prevent him from giving it to his mistress of two years?
If your husband received an inheritance, and that inheritance is maintained as separate property, that characterization doesn't change. If your husband has commingled the funds with community funds, then, the characterization changes. As separate property, he has no duty to share it.
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Family Law Attorney
If he inherited it, or if it was a gift to him, then it is his to do with it as he pleases. He could set it on fire if he chooses and there is no legal basis upon which you could object. If he transfers the money into a joint account, you may have a claim that he gifted 50% to you, but otherwise it is his to dispose of as he sees fit. If you have children and were getting a divorce, the Court could invade those funds for the payment of either child support or alimony (with child support being the more compelling reason). There really are not enough facts here to develope a strategy.
Family Law Attorney
I fully agree with my colleagues, but feel it important to add that your husband can also give his community property to his mistress or anyone else he chooses as well. Being his wife only gives you a CLAIM to half of what is acquired during the marriage. Unless you actually get divorced or legally separated, your claim is unrealized. In addition, there are some judges of the mind that if you know he is spending money on a mistress or other wasteful pursuits that do not benefit the marriage or community, you are consenting to this conduct and cannot later make a claim to recepture what he has wasted. If he has had a mistress for two years, and is still giving her things he is not willing to give you, why are you still married? If there is a reason, perhaps a legal separation would be an option. You need to consult with an attorney in your geographical area to understand your rights and how your conduct is affecting those rights.
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