The term 'CERTAIN TWIN' can be broken down into four crossword solutions: SIS (3 letters), ENG (3 letters), ESAU (4 letters), and NAN (3 letters). SIS is an abbreviation for the word 'SISTER, ' which is a female sibling. ENG is an abbreviation for the word 'ENGINEER, ' which is a person who designs, builds, and maintains machines, structures, and other items. ESAU is a biblical figure who was the twin brother of Jacob in the Old Testament. Finally, NAN is an abbreviation for the word 'NANNY, ' which is a person who takes care of children in a household. Together, these four crossword solutions form the term 'CERTAIN TWIN, ' which refers to a pair of siblings, one of whom is a SISTER and the other an ENGINEER, similar to the biblical twins Esau and Jacob.