Cómo concatenar múltiples valores de múltiples tablas de relaciones en una sola consulta mysql

Debe usar una unión diferente con rel_destinations para conseguir las ofertas con Praga como destino. Únase a esto con su consulta original.

SELECT offers.*, 
  GROUP_CONCAT(DISTINCT DEPC.name SEPARATOR ', ') AS depCities,
  GROUP_CONCAT(DISTINCT DESTC.name SEPARATOR ', ') AS destCities
FROM offers
INNER JOIN `rel_departments` ON (`rel_departments`.`offer_id` = `offers`.`id`)
INNER JOIN `departments` as DEPC ON (DEPC.`id` = `rel_departments`.`rel_id`)
INNER JOIN `rel_destinations` ON (`rel_destinations`.`offer_id` = `offers`.`id`)
INNER JOIN `destinations` as DESTC ON (DESTC.`id` = `rel_destinations`.`rel_id`)
INNER JOIN rel_destinations AS d1 ON d1.offer_id = offers.id
WHERE d1.rel_id = 1
GROUP BY offers.id

DEMO