How can I make sure my new puppy bonds with me?

A housemate began to foster two 11 week old puppies with the intention of keeping only 1. After a few days, I realized I wanted to adopt the second one (they're sisters). She has been caring exclusively for the pups for over a weeks now and they are closely bonded with her. If I adopt the second one, how do I make sure she bonds with me, without taking her away from her sister? How do we care for two puppy that we just adopted separately, but in the same household? Thanks!