Yes, I am already wondering if William was a twin brother, but although I can find Williams birth, there is no record for Isabella.
It doesn't explain why she was not on the 1851 census with them or anywhere else for that matter. She could have been in service at 9 obviously, but there is nothing coming up for her anywhere.
I have both of her marriage certificates and her fathers name is given as William Whissel on both, likewise the mother is noted as Mary nee Hepburn