Much to my dismay.... Introducing:

I'm just thinking at 1.5 lbs at 5 months, she's going to be teeny tiny. Most toy dogs finish growing at about 6-7ish months, at least height wise. Since 5 months Mia has only put on about 1 lb of weight.

i also wanted to double up on the "she may not be 5 months" part.
i worked in a puppy store and am sad to say paperwork was often faked, forged, fiddled with, even the vet used wasnt what id call trustworthy, most of the vets that work puppy stores idea of a check over is to pick the puppy up, give a quick (and i mean did you even actually listen) check of the chest and call it good.

we had puppies with obvious cold, even one with pneumonia, bad cases of worms ect all given the "a-ok" the vet would sighn off on dogs they hadnt seen ect...

the biggest thing though was the paperwork.
dogs would come in "at 8 weeks" but it was obvious much closer to 4-6 weeks, particularly the "tiny ones" its easier to sell a "teacup" for thousands of dollars if its tiny...
the paperwork all said the right age, of course, cant have the inspectors comming down on the "breeders" thne as the puppies got older, toy breeds would have their paperwork all switched around, they would lable toy puppies OLDER than they actually were because "people like smaller dogs" and its much more apealing if a 6month puppy was only a lb than an 8 week old puppy being 1lb...
lagre dogs would often be the opposite, labled younger, to apeal to that "bigger is better" crowd often attending puppystores, giants must be giants and ours were always "huge for thier age"

its sick...

if she is actually 5 months like they claim shes going to be mabe 3lbs full grown, a 3lb dog is an INSANE amount of work, shes going to be a hypoglycemia risk for the rest of her life at that age not to mention how delicate...
dodgers 3lbs, alot thing "oh so tiny he musnt be any work at all..." HA!, i have to force feed him sometimes, every HAS to be free fed so he has acress to food 24/7, he jumps off something wrong and tada broken leg, the cat could literally eat him and they CANNOT be left alone outside EVER, a 3lb dog is easily hawk food even if youve got no coyotes or bobs in your area...
im constantly asked "why dont you just let him out" people dont realize that 3lbs is smaller than a rabbit and hawks, eagnels and owls will make short work of a dog that size.
your sister has no idea what shes gotten herself into, sounds like you do though, shame that your going to end up being the primary care taker for a dog you never wanted.

that being said, shes very cute, and at least YOU know what your doing and she has the chance of a good life.

this is true.... thats why I said she might not be 5 months... I guess its hard to tell from the posted pic but she just doesnt look like she has the frame of such a small dog, although the OP did say she was underweight so that might make a difference also.

But it is very common with small dogs to SAY they are younger so people will think "oh great, they are almost fully grown" when in reality they still have a lot of growing to do. But in this case that really wouldnt be such a bad thing if they had lied... more chance for socialization, more chance for her to get bigger.

Thanks for the well wishes everyone. I do hope she's younger. That would be awesome, more time to socialize. I picked up some toys, nylabones, colostrum, tripe and a bunch of other good things. (Getting some hi-cal supplements tomorrow.) Her potty training is already proving to be tough. She's going to the vet on thursday, and if everything checks out she'll come to daycare with me and the major socialization can begin. My day is pretty much gone when I work so tomorrow and thursday she'll be relatively homebound except for walks. She is fully vaccinated so that is nice. She needs to be spayed and we'll see if she retains some puppy teeth. She probably will so hopefully my sister heard me say about dental work during her spay.. heh. She's getting a bath tonight because she stinks of Pet Store.

When she comes to daycare with me I take transit so I have access to tons of people.

potty training a yorkie is when you truly find out if you have patience or not, lol.

Lol, it makes everything harder.