Without the name of Michelle's sisters, it is even more difficult to trace a direct link.
Firstly I'm sure that you must refer to Sheffield, a city in the county of Yorkshire.
I have checked some on-line directories and found four Mountcastles listed in Sheffield.
If you want their names and addresses e-mail me on email@example.com and I will send the info to you.
You have already established that the Mountcastle family has an exclusive pedigree, my ancestors (on my mothers side) being traced back to about 1540. I have photocopies of wills dated 1679 and 1690. Also a note that some of the family prospered in the English civil war. David Miles