WARNING: Spoilers follow for Captain America #28 by Ta-Nehisi Coates, Leonard Kirk, Matt Milla & VC's Joe Caramagna, on sale now.
Aleksandar Lukin, now almost fully under Red Skull's control, and Lukin's wife Alexa have built a very influential group known as the Power Elite, allowing them to work on their real plans without worrying too much about interference.
Those plans are finally coming to fruition as they launched a terrorist attack in Central Park by releasing a dirty bomb that caused fear and horror to those it infected. To do this, the Lukins relied on a very trusted ally from his past — Red Skull's own daughter, Sin.
The issue opened with Sinthea preparing for her mission, making sure that everyone knows she is more than just Red Skull's daughter and that she is a stone-cold killer all on her own. Alexa approached Sin at this moment and said she was trying to bring out the best in the girl, helping her prove to be stronger and better than the men that she leads. They sent her into action to help fulfill Red Skull's ultimate goal of destroying the country's belief in Captain America and the dreams he preaches.
It was Sin who was there to help drive the wedge into Captain America's dreams. The Captain attacked Sin while Sharon Carter went to find the next dirty bomb. When Captain America thought he had taken down Sin, she launched a knife at him and drugged the Avenger. After this, Sin took a cheap shot and then had her men beat the Avenger into unconsciousness. Thanks to Sin's efforts, Red Skull had a video showing Captain America being beaten, with the villain explaining that this man's dreams were no longer the dreams of the people of this country.
Sinthea Shmidt made her debut in Captain America #290 by J.M. DeMatteis and Ron Frenz, showing up in that issue as Mother Superior, as she took Baron Zemo to introduce him to her father. It turned out that Red Skull was trying to produce an heir, but the child ended up a female, with her mother dying during delivery. In Captain America #298 by DeMatteis and Paul Neary, it was revealed that when she was a child, Red Skull used science to turn her into an adult with superhuman powers. She then became the leader of a group called the Sisters of Sin, all working for the Red Skull. In the end, she was de-aged into a young girl again.
S.H.I.E.L.D. took Sinthea and reprogrammed her in an effort to give her a normal life. This changed after Winter Soldier assassinated Red Skull and Aleksander Lukin had Crossbones kidnap Sin in Captain America Vol. 5 #15 by Ed Brubaker and Mike Perkins. Crossbones tortured her to break the brainwashing and she remembered who she was and became a supervillain again. She went on to almost destroy the Marvel Universe as one of the main villains in Fear Itself. Now Sin is back, once again working with Aleksandar Lukin and Alexa, doing the will of her father, Red Skull, in trying to bring down Captain America once and for all.