Learn Swedish: Min syster känner din bror. - My sister knows your brother.

Both syster (sister) and bror (brother) are en-words. Swedish possessive pronouns change depending on the gender of the noun they describe. For en-words, we use min (my) and din (your).

I learned the word vet for know. Why is it känner in this sentence?

Swedish has two common verbs for to know. You use vet when you know a fact or a piece of information. You use känner when you are familiar with a person, place, or thing. Since the sister knows a person, we must use känner.

Should it be min systern since we are talking about a specific sister?

No, after a possessive pronoun like min or din, the noun must stay in its basic, indefinite form. Even though it refers to a specific person, you do not add the definite ending to the noun. It is always min syster and din bror.

What happens to the words min and din if we are talking about multiple sisters or brothers?

If the noun is plural, the possessive pronoun changes to match it. You would use the plural forms mina and dina. For example, my sisters know your brothers would be Mina systrar känner dina bröder.