She appears to act like her mother, but reversed, she always tells Xavier and Fred to do something cool during their summer vacation rather than just sit under front of the family's digital tree. To her annoyance, they still answer her question "What are you doing?" with "Nothing" even when they are building bumper cars that move in five dimensions ("Phineas and Ferb's Quantum Boogaloo").
She appears to act like her mother, but reversed, she always tells Xavier and Fred to do something cool during their summer vacation rather than just sit under the front of the family's digital tree. She always responds, "I knew it! You guys never do anything!"
She also enjoys the same extracurricular activities as her mother: mainly, complaining about her brothers - except Amanda is annoyed by the fact that Xavier and Fred spend their days not doing anything.
Amanda looks almost exactly like her mother when she was Amanda's age, except for different clothes and hairstyle (although her skirt is pretty much the same as her mom's when she was a teenager).
"Amanda" and "Xavier" are the two names that Candace had picked out for her future children in the episodes "The Best Lazy Day Ever", "Swiss Family Phineas", and "Spa Day". Given her interest in Jeremy, it's probable that he is the father of both children. It is possible that Jeremy might be her father because her brother, Xavier, has the same blonde hair and light blue eyes as Jeremy. On that note, Amanda's face shape is more similar to Jeremy's rather than her mother's. However, this is not definite.
Candace's plan with Jeremy was that they would date through college and then marry and have children. If Amanda is approximately the same as Candace in the present day and Jeremy is Amanda's father, then her dream has come true. It was eventually confirmed by Dan Povenmire during the 2015 San Diego Comic-Con convention that Candace and Jeremy ended up staying together, further confirming that Amanda is Jeremy's daughter.
Isabella finds out that she is going to be Amanda's aunt, which means she is likely going to marry either Phineas or Ferb. However, it is possible that she just remained good friends with the Flynn-Fletcher family, so that Candace's kids call her Aunt Isabella, even though they are not actually related.