Is the oldest smarter than the younger siblings?

No, it just depends on the person, not the age or order of the siblings.

If the siblings aren't too far apart (age wise), then their IQ is about the same. If both sibling are adults (at least 25 years old), so are their brain cells are fully matured. When both siblings are young, then the age difference effects the intelligence greatly. If one sibling was 4 and the other was 7, the the 7 year old would most likely have more mature brain cells. This might be different if one of the siblings has a mental disorder.