HMO "household definition"

HMO Licensing

Southwark council have additional licensing for any HMO now (a dwelling that's occupied by three or more people in two or more households (families))

The definition of "household" is not always totally clear - I have a situation below that I believe under the legislation is a single household and not a HMO - but wanted to check -

Married couple (A + B)
Cousin (c) of A and the cousins partner (D)

SO in simple terms -
A+B married couple
C+D couple/partners where C is the cousin of A

The legislation doesn't distinguish between married and just partners, and almost all relations are classed as family - so it looks to me that this is not a HMO.
However with the penalties for non compliance I wanted to check !