could a brother donate a portion of his lung and liver to save his sister

... but I suspect her having SCLC would rule her out as a recipient, even if the operations were theoretically possible. And any transplant is going to involve immunosuppressive drugs, which would no doubt let the cancer run wild, even if she were theoretically a candidate (which again, I doubt).

(The above is a semi-educated guess.)

She is really lucky to have you as family.  I have to agree with Ex-r, the immunosuppressive drugs would rule out the surgery to begin with.  When a patient has mets, most surgical interventions are ruled out because it is assumed that there are other cancerous cells circulating.