What your SIL has told you sounds very dodgy to me. However it is worth remembering that the OB may have access to more information than us which is making him wary of encouraging the pregnancy to continue beyond Friday. The tight vagina comment is just laughable though. How does your SIL feel? Is she unhappy? If she is it is very important she seeks a 2nd opinion so she can be confident with the decision she makes, whether that be to refuse consent to the CS, or go ahead with what the doctor has recommended.

I wish her all the well in whatever she decides.