Questions & Answers about Er þetta systir þín?
Why is your translated as þín here instead of þinn?
Possessive pronouns have to match the gender of the noun they describe. Because systir (sister) is a feminine noun, you must use the feminine form þín. The masculine form þinn would be used for a brother (bróðir).
Why does þín come after systir?
In Icelandic, possessive words like your and my almost always come right after the noun. So instead of saying your sister, you literally say sister your (systir þín).
Since a sister is female, shouldn't this be in the feminine form instead of the neuter þetta?
When you are first pointing someone or something out, like asking Is this...?, Icelandic always uses the fixed neuter word þetta. You don't match the gender of the specific person for the word this in simple introductions like this one.
Are systir and þín in the basic nominative case here?
Yes! The verb er comes from vera (to be). This verb acts like an equals sign and does not force the following words into a different case. Everything stays in the basic dictionary form (nominative).