24 hour old rejected puppy... possible surrogate

I am brand new to this board, and am so glad I found it! If this is in the wrong forum, please let me know where I should repost it...

My sisters dog just had puppies yesterday, well, 3 yesterday and 2 more today! She wansn't home when she gave birth yesterday, and the mom isn't taking one of the puppies. The vet said she needed to be bottle fed and kept warm... we've gotten a bunch of supplies to do this. My sister and brother-in-law both work full-time, and since I'm a stay-at-home mom, they asked if I would take the puppy home and try to save it, so that's what I did (I live an hour away so I couldn't be there)...

We've gotten her and are feeding her and keeping her warm with a heating pad. I have two other dogs at home, and am nervous about having them around the newborn. One of the dogs, Roxanne, is a friends dog that I am indefinately dog-sitting for, and have had her since November. She's a pretty tame, well-behaved dog, but I still don't know her well enough...

She gets very excited when she hears the puppy whimper, or when she sees her. She has been spayed, so I'm not sure if that does anything when it comes to "maternal instincts". She seems to want to be close to the puppy, maybe keep her warm? We have let her smell and lick her, but then she looks likes she's getting ready to swat at her, and I'm not sure if she's trying to move the towel to be close, or if she's trying to hurt her...

Okay, so to try and make a long story a little shorter... what should I do? Should I go ahead and let her try to be with the puppy, or keep her as far away as possible? I've read that puppies need to have their mothers, if possible, unfortunatly it's not possible in this situation. I've heard of other dogs becoming surrogates, but I'm so nervous, as the puppy is SO SMALL (5 oz)!

Any advice would be greatly appreciated!