Below is the script to find the respective sid of an server proccess id.

col sid format 999999 
col username format a20 
col osuser format a15 
select b.spid,a.sid, a.serial#,a.username, a.osuser 
from v$session a, v$process b 
where a.paddr= b.addr 
and b.spid='&spid' 
order by b.spid;

SEE ALSO – Get ospid from sid