***** NOT A TEACHER *****
(1) It is almost impossible to answer some questions unless we know the context (the situation).
(2) You are a girl, after all.
(a) This could mean two different things depending how it is used in the story you are reading.
(i) A girl says, "I love flowers." You say, "Of course, you love flowers. You are a girl, after all. " = that's what we expect of girls.
(ii) A girl says, "I can't lift this heavy box." You say, "Of course, you can't lift it. You are a girl, after all." = that's NOT expected of girls.
(a) This is ONLY an example. Many girls are stronger than many boys!!!!
(3) Don't use your manners on a siren. It might kill you without your realizing it.
(a) A siren is something like a fish woman who lives in the ocean. She sings beautiful songs so that the sailors on the ships let her lead them to death. OF COURSE, this is what we call a "myth" (a story that is NOT true).
(i) So I think that maybe (maybe) it means = Don't try to be friendly toward her/it because it/she will kill you before you know what she is planning to do to you.
Have a nice day!