First, do not feel obligated at all to take a puppy from her. Be honest and upfront, letting her know you are very interested but will not purchase if you are not sure. I think anyone should respect that and not certainly not want you to buy a pup out of obligation.
Second, you may be able to get an idea of weight but no certain answer until the pup is grown. My Maddie is 11 lbs., she was a small pup until about the 9 mo mark and she hit a growth spurt. Her parents were 6 and 7 lbs.
We recently introduced a medium breed dog into our family. He will be 55-65 lbs fully grown. We are
constantly supervising their play and interaction making sure to correct any rough play. I think you can definitely make this work but you have to be willing to ensure the older dog knows how to interact with the smaller one.