It sure is cute. It doesn't seem malnourished, unless you can see its ribs (which I do not in the picture). If you can just feel the ribs, that is perfect. Puppies tend to be skinny when they are out of the roly-poly stage. The older ones don't seem obese from the photo. But your vet is the best judge of that.
Why didn't the people want it anymore? Isn't it housebroken?
Hard to tell if the older ones will come around as they seem inseparable. They might learn to tolerate the newbie, but it might never be part of their tight-knit circle. (That is what happened with my third one.) Be prepared for some assertive behavior from one of the older dogs and some shyness from the baby. It's a pack dynamic. You can't do much about that except keep alert that no one gets injured if it gets too quarrelsome.
Such a tiny pup should not be in a home with young children. It will definitely get hurt. This baby is going to need lots of very precious attention.
Good luck! 
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