Happy new year to you too!!

What Tommy777 said was about right IMO, the "immediate family" refers to spouse/partner, children, parents and siblings.
But "immediate" is obviously not your oncle, cousin, father-in-law etc.
As you noticed there is no exact rule about this, it's up to each lounge dragon to decide what is reasonable depending on how full the lounge is, how many people you are etc.
I'm sure you can manage to get a cousin inside if the lounge dragon is in a good mood.

I travelled with my parents in December and had no problem getting lounge access for all of us at ARN and LHR.
ARN was a piece of cake, I showed my BP and said I was travelling with my parents and that was it.
In LHR they were a bit more suspicious and wanted to see all the BPs, possibly to check that the family name was the same, but still not really a problem to get access.