Or something like this as only one of the queries will return data:


SELECT
  SiteKey
  , SiteName
  , ...
FROM
  fact.Sales f
  INNER JOIN Dimension.Sites s
    ON f.SiteKey = s.SiteKey
WHERE
  @SiteFullAccessFlag = 1
UNION ALL
SELECT
  SiteKey
  , SiteName
  , ...
FROM
  fact.Sales f
  INNER JOIN Dimension.Sites s
    ON f.SiteKey = s.SiteKey
  INNER JOIN #Sites ss
    ON f.SiteKey = ss.SiteKey;