Learn Italian: La moglie di Marco è tua sorella.

Normally, Italian requires an article before a possessive word. However, there is a special rule for family members: when you use a possessive adjective with a singular family member (like sorella), you drop the article. So, it is simply tua sorella.

If a wife is a family member, why does moglie have the article la?

The "no article" rule for family members only applies when there is a possessive word (like my, your, his) attached to it. Here we are saying "the wife" (la moglie), not "his wife" (sua moglie). Without a possessive word, family members still need their normal articles.

Why do we say la moglie di Marco instead of "Marco's moglie"?

Italian does not use an apostrophe-s ('s) for possession. Instead, you use the word di (of) and flip the word order. "Marco's wife" literally translates to "the wife of Marco" (la moglie di Marco).

Why do we use tua instead of tuo?

In Italian, possessive words must match the gender of the noun being owned. Since sorella (sister) is a feminine noun, we must use the feminine form tua (your).