I feel like a bad human being?

So im babysitting my sisters dogs till Christmas time. I said yes because i love dogs and i cant imagine a time i ever hated them! I love any kind of animal since I was young. Its mind blowing to me to think that i officially found a dog I HATE!!! 

This dog is a senior weenie dog and he's the cutest thing! But i swear to you know who up above that he's a full on creakhead/demon!!!! 

I gave it a treat (carefully and softly) and he full on bit me! I was giving him food in a bowl and he growled at me. My older sister got bit by him also. My mother is the only one who hasnt gotten bit and she keeps saying how wonderful he is and how she doesnt understand us. This isnt new behavior with her, shes a whole other mess that we wont get into rn. This dog has bitten me and growled at me on multiple occasions and everytime its just been me going on about my day or just ignoring him(due to not exactly enjoying his company). Ive tried to tell myself it's because hes old and has some kind of old man issues as a dog but he's oly doing it to me!!!! Ive tried everything i can think of to get his good side but this crackhead dog enjoys messing with me. Not even to mention her dogs arnt actually potty trained. This dog pisses and shits everywhere!! We've even tried taking him out every hour on the hour!!! He refuses and chooses to hold it for when hes inside!!! I cant stand him and i feel awful for feeling this way because ive always felt  "owner not the dogs fault" and i still do but i dont know what im doing wrong!! I cant pet him, i cant walk him, i cant feed him. My sisters been doing it all because hes such a crackhead!

Someone please help me understand! I know my sister(owner of the dog) isnt a bad owner! Shes a mom of 4 and they're a sweet family. I just cant understand any of it.