The Twelve Tables (c. 450 BCE) mandated that all women (excepting Vestals) have a guardian, giving their “levity of mind” as justification. At this point, women gained freedom from patria potestas (the absolute power of the head of a family) and became sui iuris in widowhood, though they still required a guardian. If an adult woman wished to change guardians, she could do so via a non-marital coemptio, or fictional sale of herself.
In 195 BCE, both Cato the Elder and Lucius Valerius in voicing their opposite opinions on the repeal of the Oppian law*** based their arguments in practice and ideal of the guardianship of women. This was still a powerful ideal, but it was changing in practice. Marriage sine manu was becoming much more common than marriage cum manu, which meant that many women became sui iuris much earlier in their lives, with the deaths of their fathers (or other guardian) rather than of their husbands.
It was only with Augustus’ laws on family and the revisions passed shortly thereafter that some women gained complete freedom from guardianship. According to these laws, freeborn women with three children and freedwomen with four no longer required guardians. By this point, guardianship was already becoming much less strict as well. There is evidence that some guardians simply agreed to most things a woman their care wanted to do.
Two centuries later, guardianship of women had become a mere formality. The jurist Gaius argued that it should be abolished entirely, since women were already managing their own affairs, and when their guardians disagreed, officials more often sided with the woman than her guardian. By the 4th century just that had happened. The requirement for the guardianship of women in Roman Law had vanished entirely.
*The only exceptions, of course, were the Vestal Virgins, in deference to their priesthood.
**That right went to whoever had patria postestas over her. A woman sui iuris was allowed make these decisions herself.
***This law was passed in 215 BCE during the Second Punic War after Rome’s defeat in the battle of Cannae. In order to ensure that more money went to the state to pay for the war, the Oppian Law restricted women’s wealth and their right to display what wealth they did have.
Women in Roman Law and Society - Diotima
On guardianship (Gaius, Institutes 1. 144-5, 190-1.) - Diotima
Marital Subordnation (Gaius, Institutes 1.108-118, 136-137a.) - Diotima
The Twelve Tables (excerpts). Rome, 450 B.C. (traditional date). (FIRA2, vol. 1, p. 23. Tr. ARS. L) - Diotima
Livy, History of Rome 34.1, exc. Late 1st cent. B.C.-early 1st cent. A.D. L - Diotima
Two contracts for the services of wet nurses for slave children. Alexandria, 13 B.C. (BGU 4.1106, 1107. G) - Diotima [Two examples of brothers acting as guardians]
Women in Ancient Rome - Wikipedia