As always, these are only sample answers.  There may be other answers that are equally good.  Let me know if there are any typos, please.

1. Cindy, who is Bobby's sister, is blond.

    Bc & Scb

2. Each of Bobby's sisters is blond.

    @x(Sxb -> Bx)
 
3. Bobby is not blond, but all of his sisters are.

    ~Bb & @x(Sxb -> Bx)

4. Alice cooks, but neither Bobby nor Cindy does.

    Ca & ~(Cb v Cc)

5. Alice, who is a maid, cooks but does not dust.

    Ma & (Ca & ~Da)

6. Maids who cook also dust.

    @x(Mx & Cx -> Dx)

7. Some maids who cook also dust.

    $x(Mx & (Cx & Dx))

8. Not all maids who cook also dust.

    ~@x(Mx & Cx -> Dx)

9. Some maids who cook do not dust.

    $x(Mx & (Cx & ~Dx))

10. All maids dust, but not all of them cook.

    @x(Mx -> Dx) & ~@x(Mx -> Cx)

11. Only maids dust.

    @x(Dx -> Mx)

12. Unless a maid dusts, she does not cook.

    @x(Mx -> Dx v ~Cx)
    @x(Mx -> (~Dx -> ~Cx))
    @x(Mx & ~Dx -> ~Cx)
    @x(Mx & Cx -> Dx)

13. None of Bobby's sisters are maids.

    @x(Sxb -> ~Mx)
    ~$x(Sxb & Mx)

14. Some of Bobby's sisters cook, but none of them dust.

    $x(Sxb & Cx) & @x(Sxb -> ~Dx)
    $x(Sxb & Cx) & ~$x(Sxb & Dx)

15. Exactly two maids cook.

      $xy(((Mx & Cx) & (My & Cy)) & (~x=y & @z(Mz & Cz -> z=x v z=y)))

16.  Bobby has at least two sisters.

      $xy((Sxb & Syb) & ~x=y)

17.  Bobby has exactly three sisters.

      $xyz(((Sxb & Syb) & (Szb & ~x=y)) & ((~x=z & ~y=z) & @w(Swb -> (w=x v w=y) v w=z)))

18.  At least two of Bobby's sisters are not maids

      $xy(((Sxb & Syb) & (~Mx & ~My)) & ~x=y)