What can I do if my brother won't let anyone see my mom, and no one else knows where she is?

Even if he is the mother's conservator it is inappropriate to refuse to disclose her location and how she is doing unless there is a reason for doing so. Conservators, sometimes referred to as guardians must abide by rules, and can be in serious trouble if there is no cause to isolate someone from family. Conservatorship is not carte blanche control over another person.

Ezcare does have a point - you need to explain the 5 w's - who what where when and how for others to respond to question.

I need a few more details. If your brother knows where she is, why won't he share that with you? Also you say "we dont know..." Are you referring to other siblings or to your husband? Finally, is your mom afflicted by dementia and does your brother have Guardianship or POA for her. If not, and you suspect that your mom may be in danger, you should immediately report her "missing" to the police and let them handle it first.