My kitties, while not related, were adopted at the same time and rarely share toys or play together (they used to, and the decided they didn't want to anymore). They've each picked out their favorite toys and don't like to share (which they've learned). Alfiq loves strings and sticks, while Kynareth loves pompoms and paper stars.
While it's disappointing they don't share or play together, they do get along just fine. They have their bickers from time to time, but that's not surprising for any two cats who live together. Cats are fast learners, and each cat will eventually learn to not play with certain toys. Of course, cats being cats, one might play with the othersother's toy because she wants a reaction from her sister. So, they may fight, but it shouldn't be bad. Just let them fight it out, or make a loud, distracting noise (like a clap), which will usually disperse the cats. Just remember, itsit's just a little sisterly love ;)