previous owner wants dog back

Question
6 months ago we were at a freinds house and one of their children came in and said does anyone want to buy a dog, we went to have a look as it was a puppy, it was  very skinny and underfed, the guy said he couldnt look after it as he already had 2 dogs, he said he wanted 40 pounds we agreed to have the dog on a trial to see if it waould get on with my other dog, we decided to keep the dog and went back and gave him the money, the dog became ill and we had to pay alot for treatment he is now doing very well on a special diet and we love him. Then the other day the sister of the guy who we bought the dog off said she wants him back and he shouldnt have sold him. we have had him 6months and bought him in good faith he wasnt being cared for properly and feel we couldnt give him back now we have put in so much time and money getting him in good condition, I dont plan on giving in that easy after 6months what are our rights  

Answer
Considering you paid in pounds, I'm guessing you're not in the States as I am.  Don't know about the laws where you are, but in America, possession is 9/10ths of the law.  Meaning, you have the dog, you have had the dog for 6 months, you paid for the dog and you put money into the dog.  I would think the only way for the sister to get the dog back is prove that the dog was stolen by the brother and sold without her consent (if she actually owned the dog).  In that case, the brother could be charged with theft.  Otherwise, if he owned the dog and sold it, you're right, done deal.

Maybe, if there's someone here who knows the laws of your area, the can chime in with an opinion.