The account number you put in is POSB account number, than you insert in POSB cheque box = Correct.
Ignore the OCBC thing as it is the bank of the person paying, not the bank where you shoud deposit the cheque to. So it don't matters much.
It's like interbank transfer like that lor. The person pays from OCBC but is deposit into POSB (your sister's account).
Don't worry since it's correct le