"Sometimes my mother talks to herself."
Translation:Soms praat mijn moeder tegen zichzelf.
The correction I received was 'Soms praat mijn moeder in zichzelf'. Please explain the 'in'.
"Soms praat mijn moeder in zichzelf" is not correct Dutch. Not sure why that correction was given to you. Edit: Per El2theK, in zichzelf praten is also correct.
Perhaps not the most common way to say it. Though it is used, e.g:
It's difficult to explain what is exactly the difference between tegen zichzelf praten and in zichzelf praten, but I suppose the former is more often used when someone is speaking aloud, while the latter refers to a person mumbling things that can either not be distinguished or do not really make sense. An actor can tegen zichzelf praten, but a philosopher who thinks aloud is in zichzelf aan het praten.
If anyone wants to add to this please feel free, but that's the way I understand it as a native speaker.