Some of the above is getting me confused!
The rule is that 'bubbles' (as the politicians and health advisors call them) or 'links (as they're actually referred to within the Regulations) must be EXCLUSIVE. If Household A and Household B form a link/bubble together, neither of them is allowed to form a link/bubble with Household C (or D or E or, etc, etc).
The 80yo in the question has already formed a link/bubble with another household (where her family members reside), so she CAN'T now form a link/bubble with the household where the 74yo resides.
Woofgang's assertion that there's no restriction on travelling together in cars is simply WRONG. A car is an 'enclosed space' as far as the legislation on smoking is concerned and it's that legislation which is then used by the coronavirus Regulations to define 'indoors'. So two or more people (from different households) in the same car constitutes an 'indoor gathering' as far as the Regulations are concerned.