As said above, I wouldn't really see too much of a problem with your cousin using it too, but you won't or atleast shouldn't play on the same server as him at the same time on the same account.
You'll get authentication errors.
Key-sharing within the household is perfectly acceptable.
Just don't let your key get shared to other people, particularly people who aren't friends or necesarilly trustworthy.
If the key gets shared between a large amount of people at different IP's and is switched constantly, as in lots of them using it around the same time, it'll get de-activated.