The Fitzgerald family matriarch is a former civil attorney with one sick daughter and… um… a few other kids that she doesn't seem to give two wet farts about.
Many of Sara's chapters are set in the past, so we get to see everything from the early days of Kate's illness up until Kate's final moments. When Kate gets leukemia, Sara feels responsible for her daughter's illness: "This is happening because I did not realize how good I have it" (1.3.103). Though there are plenty of moments when we want to smack Sara, alas, this is definitely a case of bad math on her part—she definitely doesn’t cause her kid's cancer.
Instead of doing mediation or some reiki to correct the universe, though, Sara decides to genetically engineer a child who can be a donor for Kate. (See? We told you there'd be moments where Sara's smackable.) You read that right: She makes a test-tube baby who is a genetic match for the dying Kate. She doesn't try to hide it, either; she says, "I plan for [Anna] to save her sister's life" (3.4.13). No pressure, kid.
By focusing on Kate, and only on Kate, not only do her other kids feel invisible, but it starts to put a strain on her marriage. She and Brian, her husband, barely talk about anything other than Kate's illness. Dinners are awkward affairs where she pays attention to the waiter to "keep […] from having to recognize the strangers we have become" (5.3.137). Much as we respect her dedication to Kate, Sara's really at the heart of her family's disconnection—the rest of them at least try to care about each other, while she's all Kate, all the time.
Brian actually moves out when the trial over Anna's kidney really gets going, and he has the audacity to suggest that Anna should get to decide who gets her kidney. However, at the trial, Sara finally realizes that Brian isn't on a different side than she is; they're both doing their best to be parents. And they reconcile on the stand. Well, not on the stand, but in bed after reconciling on the stand.
Sara also relents in her crusade a little bit when she learns that Kate actually wants to die. She seems to accept Anna's medical emancipation after that—but we're left seeing Sara, yet again, only accepting something because it's what Kate wants. Doesn't she see that's how they all got into that mess in the first place? Guess not.
We need to send a quick shout-out to Suzanne, Sara's older sister and "built-in best friend" (2.2.15), which is nice since Sara doesn't seem to have any other friends. Suzanne's the one who picks up the pizzas (and the pieces) for a girls' night out to take Sara's mind off Kate's illness.
She doesn't really have an impact on the plot at all, aside from that one time she offers to pay for Kate's treatment. This ends up driving a wedge between Brian and Sara, because Brian doesn't want to accept Suzanne's money. He ends up taking Kate's college fund instead, using that to pay for her treatment and angering Sara, even though Brian is probably right that Kate won't live long enough to go to college anyway.
Of course, maybe he's regretting this decision when Kate does end up living after receiving Anna's kidney after Anna's fatal car accident. But no matter. When we last see Sara, she's resorted to some sort of weird superstitious behavior, believing "that Anna would come back to her" (Epilogue.2). Maybe since her child was basically a ghost to her all along, thinking that she might come back isn't too much of a stretch for this mom.