The Helios Sisters were siblings who once invaded the Doctor's TARDIS.
After the Eighth Doctor fled from the TARDIS on Earth, the Helios Sisters possessed a boy named Peter to help them find the Doctor. Upon finding the Doctor at Peter's home, they knocked Peter's mother, Sally, out of her wheelchair. The shock let Peter shake off the Helios Sisters' hold over him. Sally then managed to absorb the Sisters' powers into herself, which subsequently killed the Sisters. (PROSE: Suns and Mothers)