Unable is the opposite of Able. Able is related to Willing. 
Son is related to Father. Father is a synonym of Dad. Dad is related to Sorry. Sorry is related to Walk. 
Unable <--Antonym--> Able <--RelatedTo--> Willing 
Son <--RelatedTo--> Father <--Synonym--> Dad <--RelatedTo--> Sorry <--RelatedTo--> Walk 