Old Norse/Grammar/Pronouns

The possessive pronoun is declined strong like an adjective:

You may occasionally see yðvar in place of yðar. This form, yðvar is the long form, and yðar is the short form for "your" (plural).

Like mín, is þín-, sín-, okkar-, ykkar-. The long vowel in mín shortens in the masculine nominative/accusative singular, and you add an -n. The feminine genitive/dative and genitive plural double the stem-ending consonant before adding the endings, explaining the double-n here, and double-r in vár. Anywhere else with a double n, the vowel shortens. Otherwise, it remains long. In vár, it doesn't shorten anywhere:

For yðar, ykkar, and okkar, you may shorten the vowel in trisyllabic forms (yðarum to yðrum, for example), but never with the long yðvar (yðar)

As a reference the possessive nominatives (masculine, feminine, neuter) are:
 * mín, þin, sín: minn, mín, mitt
 * okkar, ykkar: okkarr, okkur, okkart
 * vár, yðar: várr, vár, várt

See the pattern? Masculine nominative: double the last consonant; Feminine nominative: no change in the singular, add '-ur' in the dual; Neuter nominative: add '-t', double it in the singular and replace 'n'.