I'm stuck with a puppy that was too young to be adopted

My brother called me this morning to ask me if I could dogsit for him for a few days. Turns out that he and his wife got their son a puppy but need to keep the dog hidden until his birthday on Tuesday. I have two adult dogs and thought one more would not be a problem so I said ok. When my brother dropped off the dog he mentioned that the dog was 5 weeks, that she was a toy dog and the runt of the litter- all indications that she should have remained with her mother and siblings another few weeks. Then he said he assumed that she could eat the canned dog food he had provided. Just to be on the safe side, I bought some premixed infant formula to water down the canned dog food- or my dry dog food in case she wouldn't eat the canned. My sister noticed the pup had a flea on her face and I quickly plucked it off. My dogs have flea protection but the puppy is too young for those harsh chemicals. When doing an online search for gentle, natural flea prevention I saw information on young puppies that says not to remove them from the mother before they are about 8 weeks, not to expose them to other dogs until they have received their first set of shots, not to take them outdoors- especially in an area that has been frequented by other dogs. In short, everything that should not be done to this puppy has been done! I don't know how to fix the damage or minimize the dangers of having her here other than to keep her in a box away from my two and keep her indoors. My dogs are up to date on all vaccinations but there are lots of strays and dogs allowed to roam free in my area. I will buy some kind of oil to repel the fleas but I am frazzled and frustrated to say the least!
Please learn from their mistake and never adopt a pet before it is old enough to be separated from its mother and receive its first set of vaccinations.