Riddle 7

I wonder if you can tell
How this journey will end well
It's about a river and a boat
But no peasant, wolf, cabbage nor goat
The complication did increase
Can you, my friend, maintain the peace?

At the riverside there stand
Six different persons and
Only the grown-ups can steer the boat
"Big enough for only two", I quote
To make it simple I will name
Them as following in this game:

A does not want a to be
Without it but B or C
Unless they are of the size of a
Then being without A is okay
B feels the same about the b
And C has the same rules for c.