Jeremy Crawford clarified in the 19/JAN/2017 Sage Advice segment of the Dragon Talk podcast that a spell targets something if it affects that thing.
The Twinned Spell Metamagic states:
When you cast a spell that targets only one creature and doesn’t have a range of self... Emphasis mine
I am not sure how to interpret this section though. Does the spell need to target exactly one creature and nothing else, or can it target one creature and then also any number of non-creature things, such as objects? However, in the podcast Crawford does say fireball is not eligible to be twinned "Not only because initially you're not even targeting a creature, you're actually targeting a point in space...", and this seems to imply that my first interpretation is correct.
This matters because greater invisibility has the following in its spell description:
Anything the target is wearing or carrying is invisible...
The answers to this question about what counts as a target, seem to say that worn/carried objects are included when a spell targets a creature. Does this mean the additional sentence in greater invisibility is just a helpful reminder?