If all the baby needs is another womb (after all, we have overlooked trivial things like a blood group match etc.), then why did they not think of Sunaina? She has already had two children, she is a married woman who has no designs on Rishi, and most importantly, is family. So, the baby will be born a Sharma and will not be subject to a contract.
On another note, Shweta's explanation to Kutumb was hilarious. "Main ek doctor hoon aur patient ko bachana mera kaam hai" So does that mean that the next time she encounters a patient who needs a liver transplant, she'll donate her liver? If another of her patients needs a surrogate mother, will she do this again?? Ridiculous!