Who undertakes the marriage contract for a maiden whose father has died, and who has a brother, a paternal uncle, and a maternal uncle? And what should she do if her brother opposes her marriage so that she may serve their ill mother?

If your brother, who is your guardian in marriage (wali), refuses to marry you to a suitable suitor whom you approve of, he is considered to be "hindering" (عاضل). In this case, his guardianship is revoked and transfers to the next closest guardian in line, or to the ruler/judge. You are permitted to refer the matter to the Sharia judge, so that he may arrange your marriage or order your brother to do so.