Does my Mom have a right to see her husband"s will?

Thank you, yes my stepfather has been deem incapable by two doctors.

No, your mother has no legal right to see her husbands will. If I were you I'd be sure about the stepbrothers POA powers and also that your stepfather has been declare legally incompetent. Meaning has this been determined in court by a judge. If your stepbrother has only a doctors letter you can challenge the diagnosis if you feel it is not accurate.