The Animal Under the Fur by E.J. Mellow 

In which we have an assassin working for a secret government agency who has long red hair and lives with her best friend.  Then we have another assassin working for a sister agency.  They are sent to eliminate the same foul underworld boss.  Then they are told they must work together on an assignment to kill a drug lord in Mexico who is selling bioweapons to the highest bidder, since his original customer was killed in the prior assignment.  Only this drug kingpin turns out to be our redhead’s father.