No, it is not a probate asset. Upon the death of one member of a joint tenancy, the remaining owner(s) own the whole. Your wife can put the account in her name alone with only a death certificate. The one catch is that the Wisconsin Estate Recovery Program may have a right to funds even in a joint account if the decedent received Medicaid (medical assistance) from the State of Wisconsin.
Under the laws of most states, joint bank accounts are not considered part of the estate and pass to the surviving joint tenant.
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In general it depends - if the account was held with "rights of survivorship" or you had named each other "pay- on-death" beneficiaries, then the funds in the account should pass outside the will or probate. The bank should be able to transfer the balance to an account in your name (only) with a death certificate.
On the hand, if the the account is joint only in the sense that either of you could withdraw funds, then your spouse's share of the account would be included in her probate estate. (Her share of such account would be affected by WI marital property rules, which are similar to community property.)
Two important caveats: If your spouse had debts in excess of her probate assets, you will need to consult with a WI attorney to determine whether and to what extent the joint account is subject to such debts. Second, regardless of the type of joint account, it will be included (at least half) in the "gross estate" for federal estate tax purposes, even if it is not part of the probate estate, and you should consult a tax advisor regarding whether filing an estate tax return is advisable under your circumstances considering the portability of your spouse's estate tax exemption.
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The account, if held jointly would not a be probate asset. But the value of the your mother-in-law's share of the account, if not the whole value of the account, is part of her estate for estate tax purposes.
There is an exemption amount that may preclude any estate tax liability in this particular case.
Whoever is going to administer the estate (that person is called the personal representative, in Wisconsin) should contact an attorney, who is experienced in probate and estate tax matters.
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