Sentence construction for “whether or not”
呢就 is not a single term. They are two word particles attach to different part of the sentence

The sentence construction of ”係唔係老公老婆呢就唔知啦！"is as follow:

[係唔係老公老婆][呢], [就][唔知啦！]

[about] [are (they) husband and wife?], [just]  [(we) don't know！]

In this context: 

XXX呢 = about XXX

就YYY = just YYY

Other example:

人呢(about man), 就係生嘅 (is just alive - therefore flexible)

規矩呢(about rule), 就係死嘅 (is just not alive - therefore rigid)

租金呢(about rent), 就一人一半 (would just be half and half between us)

You can do without the particles in the sentences. That make them more direct and also more blunt: 

"係唔係老公老婆?" "唔知!"

"人係生嘅, 規矩係死嘅" (it is a very common Cantonese expression)

"租金一人一半"