Does the unmarried daughter bear the greater part of the responsibility for the care and devotion to her parents, or is this an excuse used by her brothers to avoid their responsibilities?

Filial piety towards parents and treating them with kindness is an obligation upon all children, married and unmarried, due to the generality of the legal evidences. It is one of the greatest avenues for Allah's pleasure and entry into Paradise, and all children should compete in it. The care of parents is not limited to the unmarried; rather, it is a collective responsibility, and if one of them becomes unable or needs assistance, then others must bear their responsibility and share the service or bear its costs. Children must do this willingly and hoping for reward, and resolve disputes with gentleness.