"Kateřina and Žofie are Matěj's aunts."
Translation:Kateřina a Žofie jsou tety Matěje.
Under what circumstances is it ok to turn one's name into the possessive (rather than of insert name)? I hear it done all the time, and I tried here "Kateřina a Žofie jsou tety Matějové", but it was counted wrong. I don't see it explained anywhere, but maybe I haven't gotten there yet
Here "Matějové" is just wrong. The correct feminine possesive is Matějovy.
There are two kinds of adjectives in Czech.The so-called agreeing and non-agreeing ones. The agreement is in declination.
Non-agreeing is "Matěje" because Matěje does not change:
gen. tet Matěje, dat. tetám Matěje, acc. tety Matěje, loc. tetami Matěje
Agreeing is "Matějův, Matějova, Matějovo". It does change together with the noun and agrees with the noun in the grammatical case.
gen. Matějových tet, dat. Matějovým tetám, acc. Matějovým tetám, loc. Matějovými tetami
The agreeing adjective shoul normally be placed before the noun and the non-agreeing one after the noun.
This rule is often not followed for the non-agreeing adjectives when naming commercial products and locations, but I personally find it very bad and against the language. Here the rule should be followed.
For agreeing adjectives there are exceptions to this rule for certain names especially in biology but also in poetry, in certain archaic use cases and elsewhere. But here "tety Matějovy" would be incorrect, way too formal if you would apply the exception.