It may not be exactly what you are requesting, but here is a start. Ask if you have any questions.

select

sp.name,

spa.name

from

sys.server_principals sp

inner join sys.server_role_members srm

on (sp.principal_id = srm.member_principal_id)

inner join sys.server_principals spa

on (srm.role_principal_id = spa.principal_id)