She appears to act like her mother, but reversed, she always tells Xavier and Fred to do something cool during their summer vacation and not 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 and not 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 activities as her mother: mainly, complaining about her brothers - except Amanda gets 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, as seen in "The Best Lazy Day Ever", "Swiss Family Phineas", and "Spa Day". Given her interest in Jeremy, it is highly likely that he is their father. Genetic evidence supports this as well: her brother, Xavier, has the same blonde hair and light blue eyes as Jeremy.
Amanda's, Xavier's, and Fred's situation is analogous to Phineas, Ferb's, and Candace's except rather than trying to "bust" her brothers for doing something, Amanda tries to bust Xavier and Fred for doing nothing.
Isabella finds out that she is the future aunt to Amanda, which means she is likely going to marry either Phineas or Ferb.
The episode "Act Your Age" confirmed that Phineas and Isabella start dating in the future. Thus, it is almost certain that she will marry Phineas and not Ferb.