News about James Gunn’s involvement and new vision have only begun trickling out, so smart money says its too early to nail down something like a part for his brother.
Being related to the writer/director would presumably grant Sean with access to early information, including information regarding a possible role.
Sean Gunn’s willingness to join The Suicide Squad is very exciting, although it seems that the villain-centric DC blockbuster is still in the early stages of development.