Early 1800s "step sister"?
Solution 1:
The earliest match that I've been able to find for step-sister is from André Favin, The Theater of Honour and Knight-hood. Or A Compendious Chronicle and Historie of the Whole Christian World (1623):
Anne, Duchesse of Bretaigne, was by the Testament of the last Duke Frances the Second her Father, left in the Guard and Tutelage of the Lord de Rieux, Marshall of Bretaigne, and of the Lady de Loual, Step-Sister to the Lord Allaine d'Albret; in regard that the said Anne was then but twelue yeares old, being borne in the yeare, One Thousand, Foure Hundred, Three Score and Sixteene.
Samuel Johnson, A Dictionary of the English Language (1755) observes that step in the kinship sense had been used by numerous authors:
STEP, in composition, signifies one who is related only by marriage. {Steop, Saxon, from stepan, to deprive or make an orphan : for the Saxons not only said a step-mother, but a step-daughter, or a step-son; to which it indeed, according to this etymology, more properly belongs : but as it is now seldom applied but to the mother, it seems to mean, in the mind of those who use it, a woman who has stepped into the vacant place of the true mother.}
[First cited example:] How should their minds chuse but misdoubt, lest this discipline, which always you you match with divine doctrine as her natural and true sister, he found unto all kinds of knowledge a step-mother. Hooker [Of the Laws of Ecclesiastical Polity (1594).
[Second cited example:] His wanton step-dame loved him the more; / But when she saw her offered sweets refuse, / Her love turn'd to hate. Spenser [The Faerie-Queene (1650)].
[Third cited example:] You shall not find me, daughter, / After the slander of most step-mothers, / Ill-ey'd unto you. Shakespeare [Cymbeline (1611)].
[Fourth cited example:] A father cruel, and a step-dame false. Shakespeare [Cymbeline (1611)].
[Fifth cited example:] Cato the elder, being aged, buried his wife, and married a young woman: his son came to him, and said, Sir, what have I offended, that you have brought a step-mother into your house? The old man answered, Nay, quite the contrary, son; thou pleasest me so well, as I would be glad to have more such. Bacon. [Apophthegms (1625)]
[Sixth cited example:] The name of step-dame, your practis'd art, / By which you have estrang'd my father's heart; / All you have done against me, or design, / Shows your aversion, but begets not mine. Dryden [Aureng-Zebe (1675)].
[Seventh cited example:] A step-dame too I have, a cursed she, / Who rules my hen-peck'd sire, and orders me. Dryden [Virgil's Pastorals (1697)].
[Eighth cited example:] Any body would have guessed miss to have been bred up under the influence of a cruel step-dame, and John to be the fondling of a tender mother. Arbuthnot [The History of John Bull, Part II (1712/1727)].
Although Johnson says that, in his day, use of the term step to identify kin by marriage is mainly limited to the word step-mother, other kin, before and since, have been also identified as step- relatives, including step-sisters. Indeed, A.L. Mayhew & Walter Skeat, A Concise Dictionary of Middle English from A.D. 1150 to 1580 (1888) confirms that (as Johnson's remark associating step with orphan suggests) the primary step kinship terms in Middle English involved orphans:
Step-barn, sb. orphan, H.—Cp. AS. step-cild, orphan, Ps. 67. 6 (VP); AS.stéop orphaned; cp. OHG. stiuf.
Noah Webster, A Dictionary of the English Language (1828/1832) sums up the contemporaneous understanding of step and step-sister (and demonstrates his familiarity with Johnson's dictionary) in two concise entries:
STEP, Sax. steop, from stepan, to deprive, is prefixed to certain words to express a relation by marriage.
...
STEP-SISTER, n., A sister-in-law, or by marriage, {an orphan sister.}
But the definition that Webster gives is a dual one: elsewhere in the same dictionary, Webster reports that a sister-in-law is "A husband's or wife's sister," whereas an orphan is "A child who is bereaved of father or mother or of both." So a step-sister might refer to the sister of a person's spouse or to the spouse of one's brother (whether that woman's parents are living or not), or to a girl or woman whose surviving parent has married a person's surviving parent. Evidently, divorce was sufficiently rare that it did not figure into Webster's definitions.
A clear and consistent distinction between sister-in-law as sister of a person's spouse (or spouse of a person's brother) and step-sister as quasi-sibling of a person by marriage of their surviving parents did not emerge for some time. The 1847 edition of Merriam-Webster's A Dictionary of the English Language replicates the wording of the 1828 edition's definition of step-sister, and the 1864 edition has this ambiguous entry:
Step-sister, n. A sister by marriage only.
Again, the key unanswered question here is "Whose marriage?" Finally, Webster's International Dictionary (1890) offers a breakthrough in clarity:
Stepsister, n. A daughter of of one's stepfather or stepmother by a former marriage.
Here at last we have definition of stepsister that doesn't overlap at all with the definition of sister-in-law. Prior to that, step-sister was definitely a kinship relationship based on marriage, but it did not make clear whether the marriage in question involved a person's parent or a person's sibling (or the person him- or herself).
In the case of the 1804 will that the poster asks about, without knowing the family trees involved and the name of the "step-sister" who was a beneficiary of Mary's will, we can't say with confidence whether the beneficiary was someone we would describe today as a step-sister or as a sister-in-law. Neither possibility seems at all far-fetched, for which reason a careful lawyer dealing with a family full of people named Elizabeth might have advised Mary to specify that the intended beneficiary was her brother's wife Elizabeth or her husband's sister Elizabeth or her stepmother's daughter Elizabeth.
Solution 2:
The expression is from much earlier than the 19th century:
step-sister (n.):
also stepsister, mid-15c., from step- + sister (n.).
(Etymonline)