Get parent/child or parent/siblings depending on the ID passed
Your first query can be simplified to:

SELECT a2.id, a2.title 
FROM article a2
WHERE a2.parentid = 61


To combine this with your second query you can use union:

SELECT a2.id, a2.title 
FROM article a 
JOIN article a2 
    ON COALESCE(a2.parent_id, a2.id) = a.parent_id 
WHERE a.id = 61 AND a2.id &lt;&gt; 61

UNION

SELECT a2.id, a2.title 
FROM article a2
WHERE a2.parentid = 61;