Each of the letter pairs below can be followed by the same three letters, in the same order, to make a  word.

AB
CR
EL
EN
GR
IR
OR
PL
SL
ST


Answers:

The letters are ATE:

ABATE
CRATE
ELATE
ENATE (related on the mother's side)
GRATE
IRATE
ORATE
PLATE
SLATE
STATE