1. "Who" is a subject pronoun and "whom" is an object pronoun. When in doubt, substitute a subject pronoun (he, she, they, etc.) or an object pronoun (him, her, them, etc.) for who/whom and see which one makes sense.
[Who or whom] is sitting with Martha? (HE is.) Therefore, you would use the subject pronoun, "who."
Three women are talking in the park. One of [who or whom] is my sister. (One of THEM is my sister.) Therefore, you would use the object pronoun, "whom."
Many times you will see "whom" accompanied by prepositions like to, for, of, with, or about. (to whom, for whom, of whom, with whom, about whom.)
2. "Are not" and "is not" can be made into contractions two ways. Both are correct.
Are not = 're not OR aren't (We're not hungry OR we aren't hungry.)
Is not = 's not OR isn't. (She's not home OR she isn't home.)