It will be the first time since 1998 that the Williams sisters have played at the same international-level tournament, other than the majors.
Venus Williams has competed at the event since 2014.
She won the trophy in 2015, beating top seed Caroline Wozniacki in the final, and came runner up in 2014.
This year she was upset by young Russian player Daria Kasatkina in the first round.
Tournament Director Karl Budge said he was overwhelmed to have Venus join Serena at the 2017 tournament.
