SELECT
  f.*,
  m.*
FROM
  students st
  LEFT JOIN parents f ON f.id = st.f_id
  LEFT JOIN parents m ON m.id = st.m_id
WHERE
  st.id = 333

SELECT
  st.*
FROM
  parents p
  LEFT JOIN students st ON (p.id = st.f_id OR m.id = st.m_id)
WHERE
  p.id = 333

SELECT
  sibl.*
FROM
  students st
  LEFT JOIN students sibl ON (sibl.f_id = st.f_id OR sibl.m_id = st.m_id)
WHERE
  st.id = 333