Translation:My sister often thinks about her cats.
perhaps you can expand on that for me - is this one ,,swoich" then because there are multiple cats? I put "thinks about THERE cats" but it was rejected (probably because its the wrong Their/there) as I thought there was some inferred context she shares the cats with someone else.
Why isnt this Swoje in the question in this case?
Yes, she has more than one cat. There are no other people in the context of this sentence so swoich means "her own." Swoje koty would use the nominative case if it were the subject of the sentence. The reason why it's swoich kotach is "to think about [something]" in Polish is myśleć o [czymś]. The [something] takes the locative case. In this example, czymś is the locative case of coś meaning "something."
I've compared those two structures in the Corpus of Contemporary American English:
verb + preposition + any word + noun + often + full stop
often + verb + preposition + any word + noun + full stop
That's a ratio of 1:36, hence I would call 'put modifiers at the end often' a bit of an overstatement. But, ok, let's accept it here.
Edit: Oh, wait, it has already been among the accepted answers. I guess you must have made another mistake somewhere and didn't notice it.