The earliest woman novelist of significance in the 18th century is :

Aphra Behn is the intended early woman novelist, but she died in 1689, so the question incorrectly places her in the eighteenth century. Mary Edgeworth apparently intends Maria Edgeworth, whose first novel appeared much later; Mary Russell and Mrs Gaskell also do not supply an early eighteenth-century alternative. The period error prevents an unqualified correct answer.