(1) Here, "only" is adverbial, so I would use только (or лишь would work here too, I think), and place it immediately before the direct-object phrase:
Кошка любит только мою сестру.
The cat loves ONLY MY SISTER. (The cat hates me, my brother, and my parents.)
I would ask a native speaker, but I'm pretty sure that you could also put мою at the end to emphasize it:
Кошка любит только сестру мою.
The cat loves only MY sister. (The cat hates Olga's sister, and also Boris's sister.)
Contrast this with:
Только кошка любит мою сестру. = Кошка одна любит мою сестру.
ONLY the cat loves my sister / The cat ALONE loves my sister (the dog hates her, and so do my parents, and so do I!)
(2) The adjective единственный is better used to translate "the only one" or "the sole example of," etc.
Самка пятнистой гиены (вида Crocuta crocuta) -- единственное млекопитающее женского пола, у которой "фаллос" имеется!
The female of the spotted hyena (species C. crocuta) is the only mammal of the female sex that has a "phallus"!
29saw_drawing-blog480.jpg
(Yes, that's a baby hyena in her belly, and yes, мама-гиена рож(д)ает детёныша сквозь клитор. Possibly this explains why female spotted hyenas are much more aggressive than the males of the same species...))))