[whswhs]

Well, for the heiress, here's how it works in GURPS Locations: Worminghall for college students in the Middle Ages:

Most students at Worminghall have no jobs and aren’t self-supporting.
The usual rules on economic matters – for example, income and starting
wealth – are an awkward fit. A different interpretation works better for
those who are economically dependent.
A dependent person’s Wealth level represents the
resources of his family, not personal wealth. He shares in his family’s home,
clothing budget, and similar assets, rather than having his own. He has
access to 20% of the usual starting wealth as discretionary funds, for such
things as grimoires or nights at the tavern.
However, he doesn’t have to earn his own income; his parents or
guardians support his living expenses and his studies. If he spends summers
at home working, they can contribute 1/2 of typical monthly pay for
their Wealth level. If he doesn’t, they can contribute 1/5 of typical monthly
pay for their Wealth level. If the student spends the summer with his family,
he doesn’t get money for living expenses during the summer; he shares
in their lifestyle and is included in their cost of living.

As for Social Stigma, a woman in the 1920s is not Valuable Property, but you could make a case for Second-Class Citizen, in that it's likely enough that men condescend to her, not all jobs are open to her, and so on. (A woman with a certain kind of reputation would belong to a Minority Group, but that doesn't apply here.) I think you could also say she's a Minor, in that she's not economically independent, and doesn't have full citizenship until she's 21—and she might still be economically dependent after that.