I'm not sure specifically as there could be many novels with such a title. It might be about a hidden relationship between sisters, perhaps some secrets that are gradually uncovered regarding their family, past events, or personal lives.
A 'secret sister novel' could be about a hidden or unknown sister relationship. It might involve themes like family secrets, sisterly bonds that are suddenly revealed, or the impact of a long - kept sister relationship on the characters' lives.
Since I haven't read it, I can't say for sure. But likely the sisters would be main characters, perhaps their parents or other family members could also be important characters in the story.
Typically, the main characters would be the two sisters involved in the secret relationship. There might also be their parents or guardians who are somehow related to the secret.
I'd say no. 'A Sister's Secret' sounds like a made-up tale. Authors often come up with intriguing names to captivate readers and tell fictional stories that explore various themes and emotions.
In fanfiction, the secret sister might impact the MCU plot in various ways. She could be a key to unlocking some hidden secrets about Tony's family or Stark Industries. Her existence could create a divide within the Stark family if some members knew about her and others didn't. This family drama could then spill over into the larger MCU world. For instance, if she was experimented on as a child, like Tony was in some ways, she could have developed unique abilities that could be a game - changer in the fight against powerful villains. Her introduction could also lead to new alliances and rivalries within the MCU.
One interesting Secret Sister story could be about two sisters who secretly write letters to each other every week, sharing their deepest dreams and fears without their parents knowing. It becomes a special bond that they cherish.