If the plane gets a field position after landing at Frankfurt Airport all passengers and the crew will be brought to the terminal by bus(es). In the terminal building follow the signs to passport control, baggage claim and finally customs / exit.
Your cousins will be waiting at the exit in the arrival zone to which you will be guided by the signs. The arrival areas (one per concourse) is in the terminal building - one level below the departure area in terminal 1 (T1); and at one central exit at T2.
Just to mention: the airport has also meeting points where people can find each other but for picking up passengers it s more easy to wait at the exit.