Sign is related to Aid. Aid is a synonym of Sustain. Sustainable is derived from Sustain. 
Loose is a synonym of Tight. Tight is the opposite of Stretch. Stretch is related to Length. 
Sign <--RelatedTo--> Aid <--Synonym--> Sustain <--DerivedFrom-- Sustainable 
Loose <--Synonym--> Tight <--Antonym--> Stretch <--RelatedTo--> Length 