Mothers have a tendency to give attention to their children and to others that appear to have been neglected. In this case the mother happens to be a cat, which tend to stick to their own litters and nothing else. In fact, cats can be so picky that it can be hard to predict their behavior at times. In this case however the cat adopts an orphaned rabbit, which seems a little odd considering that cats are predatory animals.
This cat however understands that the baby bunny needs a mother as it seems to be missing hers. The poor mother bunny was killed and left the baby behind. The mother cat though already has a litter of her own and seems to have plenty of milk to provide for the baby bunny. She carries the bunny like she would her own kittens and keeps them all together, so all the other brothers and sisters will also see the baby bunny as their sibling. This mother cat seems content and has no problem adding a little extra bundle to her group. The bunny gets right in with the rest of the group when it is time to eat and the mother gives her a bath too.