I’m very glad you wrote. Sitting with questions like these all that time must have been very hard. It’s very normal to have sexual feelings and curiosity at your age. It’s not at all unusual for boys to check each other out and to masturbate together in their early to mid teens.
What could get you in trouble, though, is the age difference between you and your cousin. I realize it just got going because you hang out together a lot. But it would be wise to stop this activity with him unless you know for sure what the laws of your state have to say about your ages.
For more detailed answers to your questions about sex, you need to be able to have a conversation, not just the advice I can give in a letter. The San Francisco Sex Information Line provides free counseling in all aspects of human sexuality. They are confidential and open-minded. Call 415-989-SFSI Monday – Thursday 3:00 – 9:00 p.m., Friday 3:00 – 7:00 and weekends 2:00 – 6:00 PST. Really. You can ask them anything and they’ll give you an honest answer.
I wish you well.