Only half because her dog is bigger than dd2's cat so it is not fair if dd2 gets a whole dog in return for one cat 
It recently came to dd2's attention that dd1 has a dog (Whippy) DH has a dog (the grund) and I have a dog and a cat. Dd2 has nothing. Some quick thinking lead me to tell her "Oh but the cat is yours, dd2. We knew you would like cats before you were even born so we bought one ready for you coming" This helped to explain why dd2's cat has been with us for 11 years and dd2 is only 5 
She was very happy with this and relished the role of cat mother, brushing him daily and always making sure he had fresh water and laying his food down for him. She has even assisted with de-fleaing and de-worming him (he is a very biddable, non scratchy cat)
Dd1 has noticed that dd2 is able to have a much more active role in looking after her cat than she does with her dog. She must have come to the conclusion that this is because cats are somehow to easier care for than dogs and not because when we tell her "dd1, your dog could do with a brush" her reply is always "In a minute" so we stopped asking her to care for her and we took over.
The realisation that dd2 takes a more active part in caring for her pets because she enjoys it came to her when dd2 told her "Your cat has not been brushed today and his water isn't cold still, you need to do something about that or I will tell mammy and she will give me the cat back, you can't have him if you won't look after him properly and I need to know which half of the dog is mine because she needs brushing and if I own the head end her water is not cold either" 
Dd1's reply to this was of course "I'll do it in a minute. I'm just doing this game first".
Dd2 has now asked if she can have both the cat and the whole of the dog. She doesn't think dd1 will even notice 