As always, these are only sample answers.  There may be other answers that are equally good.

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)