Sounds like a bad situation all around. If your mom got rid of your snakes once, I wouldn't personally feel comfortable buying more and putting them back into the same home they were in before for fear of them meeting the same fate. As for the dogs, it seems like your mom's boyfriend cares about himself and his dog and doesn't care what if anything happens to your animals or your sister's. Also, with a wound like that, I'm sorry, but your mom saying that she didn't "notice" the dog was bleeding was BS unless she went out of her way to NOT see it...

My opinion would be to find someone, anyone, who can take care of that poor dog and find him/her a new home, even if temporarily. If left in the same environment with the same uncaring individuals, there's no doubt in my mind that the dog will be neglected, the wound won't be cared for and may become infected and make the dog sick. If you truly love and care for the dog, get him/her out of the situation just like you got yourself out, by any means necessary...

Oh, and as for dog bite scars, they suck, but I wear mine without shame... Mine are on my head, so they're a bit harder to cover... German Shepherd got me when I was 3 years old and now 29 years later, the physical scars have never faded...