How do I explain the bris to his older sibling(s)?

Try to focus on the positive, explaining that this is a very special ceremony for your brother to connect with his Jewish heritage. All Jewish boys have a bris. There is a wonderful paperback book you can get at most Jewish book or gift stores to read to older siblings. It's called, "Rosie and the Mole. The Story of a Bris." It explains the bris ceremony in words and pictures that children from 2 to 15 should enjoy. It is written by Judy Sliverman, and was published by Pitsopany Press in 1999. The publisher's address is 40 East 78th Street, Suite, 16D, New York, NY 10021. They can be reached at 1-800-232-2931 or at pop@netvision.net.il . The ISBN number is 0-943706-20-3. I highly recommend it!