Yes, it would be, even though you did not instigate it. You have told us your size and that you are huge. You have also told us that you jack-off with your brothers. But, a 13 year old cousin. No way, man. Don't get caught in that practice. It could just develope into something you don't want to be involved in.
I am certain, that after your young cousin saw it, he was not only curious, but wanted to see and know more. But, tell him NO the next time he asks, and make it a definite NO and not a MAYBE. Tell him that you do not want him to ask again.
But, don't make it a mystery either. Don't be afraid to let it be seen in normal conditions. Just don't go out of your way for it to be seen by him. There is plenty of time for him to discover these things when he becomes older.
If you are on good relations with his father, you might want to say that you think it is time for a father and son talk. This is not a talk that should be given by a cousin.