Possible Clues:

• A barrel with chlorine for city in Greece or Bulgaria, say
• Grandma's brother
• Relative origin of universe bounded by morphing rectangle
• A cruel gent that's disturbed aged relative
• Terrific relative?
• One of those Blood Brothers, with close ties to mum's mum is a family favourite
• Old relative of a cruel gent? Wrong!
• One who should be avuncular could be a cruel gent