How does the family obligation to get married affect the Bennet sisters in Pride and Prejudice? What effect does this have on them as characters,...

Jane Bennet feels the pressure most strongly.  As the eldest daughter, and -- as her mother tells her often -- the most beautiful, her chances of making a good match are probably the best.  When Mr. Bingley finally proposes to her, she tells Elizabeth that she must go "'instantly'" to her mother, and she is most happy about the fact that "'what [she] has to relate [to Mrs. Bennet] will give such pleasure to all [her] dear family.'"  She knows that her mother will be thrilled about her engagement and that her marriage to Bingley will mean that her mother and sisters always have somewhere to go after Mr. Bennet's death.  Jane is so elated that she wonders "'how [she] shall [...] bear so much happiness."  Her relief and elation over the fact that she will marry well indicates the extent to which she's internalized and been affected by the pressure put on the girls to find rich husbands. 

As far as the other sisters, Kitty and Lydia have "minds [that] were more vacant than their sisters'," and the pressure for at least one of the Bennet girls to marry well really doesn't seem to have any effect on them.  They are more interested in flirting and balls than serious engagements and would be happier with the attentions of a handsome officer than the affections of an unattractive gentleman.  The fact that Lydia willfully elopes with George Wickham, a poor soldier, proves this.  Nor is Mary particularly affected by a sense of family obligation in marriage.  In fact, Mary seems to have no interest in men or marriage whatsoever, and because she is plain, there is little expectation that she will be the one to save the family. 

Elizabeth is also relatively unaffected by the pressure on the girls to marry well.  She cannot bring herself to value wealth over love and respect, as is clear from her initial rejection of Mr. Darcy's marriage proposals, proposals which she can only accept in the end, after she has developed both love and respect for the gentleman.  Her rejection of Mr. Collins's proposal, despite the fact that he will inherit Longbourn when Mr. Bennet dies is further proof of this.  Elizabeth even tells him, "'To fortune, I am perfectly indifferent [...].'"  Her mother obviously puts a great deal of pressure on her to accept this proposal, refusing to speak to Elizabeth ever again if she persists in rejecting Mr. Collins, but it does no good. 

Thus, Jane is really the only daughter who feels the pressure keenly.  She certainly doesn't marry Bingley because he's rich and can help her family, but her relief that she has fallen in love with a rich man is apparent.  Though Charlotte Lucas advises Jane (via Elizabeth), early on, to be more forward with her affections in order to secure Bingley's, pressure to marry well never does compel Jane to act in a manner that her conscience couldn't support.  Therefore, she does feel the pressure most deeply, but she has too much integrity to allow it to dictate her actions.