If you wiosh, you couldtry to follow both John Thomas and his wife Mary back, using the censuses, to their childhood, to see what siblings each have, and then follow each sibling forward, and using the marriage index too, see if any married a Robinson, thus being likely parentrs for Muriel.
If you need help just ask, but if you have full Ancestrry sub, then you have the same records at your disposal as I have.
Even if this method comes up with a strong inkling as to Muriels parentage, it would still be best to get the birth cert to confirm.
