Breakdown of Mi hermana se peina en el cuarto.
la hermana
the sister
en
in / on / at
el cuarto
the room
mi
my
Questions & Answers about Mi hermana se peina en el cuarto.
Why do we need se in this sentence?
Se is a reflexive pronoun. We use it because the action of combing is something your sister is doing to herself. The base verb is peinarse (to comb one's hair), and since 'my sister' is a 'she', we use the third-person pronoun se.
The English translation says 'her hair', but I don't see the Spanish words for that. Where is it?
In Spanish, when you use a reflexive verb for a daily grooming routine, you often don't need to state the body part if it's obvious. Se peina translates literally to 'she combs herself', which naturally implies she is combing her hair.
Can I put the se after the verb, like Mi hermana peinase?
Not in this sentence! When the verb is conjugated (changed to match the subject, like peina matching 'my sister'), the reflexive pronoun must go right before the verb. You only attach it to the end when the verb is in its dictionary form, like peinarse.