S.S.
When my daughter became a big sister, I always appreciated when she also got a present. It didn't really matter what it was. It didn't need to say "I'm a big sister" on it. She did get one shirt that stated "I'm a big sister", which she loved. However, it was really the thought that counted. She loved getting something and it made her feel special when her baby sister got presents. I typically give the older child a book, such as Dr. Suess or by another children's author.
It is very sweet and thoughtful of you to think about the older son!