Wouldn't swojej fit here?
I think 'My sister thinks about her cats often' should be accepted
I got this wrong ... ;) what would the ending have been for 'her cat'
I wrote 'her cat' which was incorrect. I was asking how 'her cat' (singular) would have been written. Sorry, I didn't make this clear enough.
"... jej kocie" for "her cat"
Thank you ;)