Brother died. We can't find his will. It said everything was to go to his 3 siblings, all agree. Mt law says it goes to mom. Mom wants us to get it.

First, you need to get someone appointed as personal representative of your brother's estate.  It sounds like your mother probably has priority to be appointed as the closest heir, and then each of the siblings has equal priority.  The probate court has forms for someone with priority to "renounce" - they waive their right to appointment and can agree to someone's else's appointment.  If you all agree who should be appointed, that person can file and everyone else can provide signed renunciations.   If all heirs agree, your brother's estate can be settled however they agree.  Once a personal representative is appointed, you can put the agreement in writing and have your mother and all children sign it.  This is assuming that your brother does not have children, who would also be heirs, and that your father is not living.  Another option is to have your mother sign a "disclaimer" stating that she does not want the inheritance.  She may want to consult with a tax or elder law attorney as refusing an inheritance can have tax and Medicaid nursing home assistance consequences.