Learn Hardcore Norwegian: Søsteren min studerte i utlandet i to år og lærte mye om seg selv. - My sister studied abroad for two years and learned a lot about herself.

om seg: grammatical, but in practice we almost always say om seg selv to mean about oneself . Selv adds clarity and emphasis.
om henne: means about her (some other woman) , not reflexive. Hun lærte mye om henne = She learned a lot about her (another woman) . Here we want it to refer back to the same person, so seg selv is needed.

Breakdown of Søsteren min studerte i utlandet i to år og lærte mye om seg selv.

Why is it Søsteren min and not min søster?

Both søsteren min and min søster are grammatically correct, but they sound a bit different.

søsteren min
- Literally: the sister mine
- This is the most natural everyday way to say my sister.
- Structure: søster (sister) + -en (definite ending) + min (my).
min søster
- Also means my sister.
- Slightly more formal / written style, or used when you want to contrast or emphasize the possessor:
  - Min søster, ikke broren min, studerte i utlandet.
    (My sister, not my brother, studied abroad.)

So in normal, neutral speech, søsteren min is more common than min søster in Bokmål.

What is the function of the -en ending in søsteren?