I'm guessing that dad took the dog on very reluctantly - "My dad doesn't really care about animals.." - so i have to ask, what did you and your mum expect to happen with the dog under his care?
You are the one who took the stray pup in. Did you not stop to think that it would grow in size? The dog is your responsibility and it should be you who decides what to do from hereon in. As has been suggested, find the dog a new home where she will be properly cared for and loved.
And the next time you come across a cute little stray, stop and think.