Your husband sets out in his will how he wants his assets to be distributed after his death. Not all assets pass under a will; some property, like insurance policies and investment accounts, can pass to a designated beneficiary independent of the will. Whether a beneficiary can assign his share of a trust depends upon the language of the trust document.
Assets Passed by WIll
When your husband dies, his estate passes to those beneficiaries named in his will. All property owned in his individual name passes under the will, other than assets that pass under contract to a named beneficiary. Generally, an estate is administered by the executor in a court-supervised process called probate, where assets are collected, debts paid and beneficiaries located.
If a person is the beneficiary of a trust, the status of his interest in the trust after his death depends on the terms of the document. Some trusts are written to provide benefits to someone for his lifetime only; on the death of the beneficiary, the trust benefits or assets pass to another person named in the trust. Other trusts allow the beneficiary to name a successor beneficiary in his will. If that is the case with your husband's trust, he may be able to assign his share of his mother's trust to others through his will.