within a specific DB role?
For database roles & members you have to use the database DMV instead of server DMV's =>
select *
from sys.database_principals as roles
inner join
sys.database_role_members as drm
on roles.principal_id = drm.role_principal_id
inner join
sys.database_principals as mem
on drm.member_principal_id = mem.principal_id
where roles.type = 'R'