Breakdown of Der Geburtstag von meiner Schwester ist im Januar.
sein
to be
die Schwester
the sister
der Geburtstag
the birthday
von
from, of / by (agent in passive clauses)
Questions & Answers about Der Geburtstag von meiner Schwester ist im Januar.
Why is it meiner Schwester and not meine Schwester?
The preposition von (of/from) always forces the following noun into the dative case. Because Schwester is a feminine noun (die Schwester), the word meine (my) changes to its dative form, meiner.
Is von meiner Schwester the only way to say "my sister's"?
No! You can also use the genitive case, which is specifically for showing possession: Der Geburtstag meiner Schwester. Using von plus the dative case, like in this sentence, is a very common and slightly more conversational way to say the exact same thing (literally "the birthday of my sister").
Why doesn't German just use an 's' like English, such as "meine Schwesters Geburtstag"?
In German, you can only add an 's' to show possession if you are using a proper name, like Marias Geburtstag. For regular nouns like Schwester (sister), you have to use the structure in this sentence or the genitive case.
Why do we say im Januar instead of just in Januar?
The word im is a contraction of in dem (in the). In German, all months are masculine and require the definite article when you say something happens in them. Because in triggers the dative case when talking about a fixed time, der Januar becomes dem Januar, and in dem shrinks down to im.