I am having the second child. Should my first child attend sister/ brother class?

Not necessarily. I think it depends on the child and the age of the child.
Usually not needed. A first child usually doesn't need to attend a brother/sister class in order to prepare for the arrival of the sibling. Explaining to child #1 that the family will be growing (just like other families), that more children leads to more fun & excitement for all, and that he/she will be a helpful big brother/sister will help child #1 adapt. Some helpful books for kids and parents aid the transition.