New seats are height based not weight based.
Legally he should still be in a seat with inbuilt 5pt harness. An add on harness with a booster does not satisfy this legal requirement, as well as not being recommended due to potential safety issues -refer to sticky at top if this forum about safety concerns over harnesses with booster seats.
An old up to 18 kgs seat was tested with 22kgs test dummies, so is fine to use to 22kgs. Current height based marked seats will suit children up to over 120cm in height depending on the seat. Most seats allow for children to 110cm and the smaller ones to around 100cm. So there is a vast difference. You may have issues legally arguing that your child was too big for a seat if they are only 100cm.
So unless you nephew is over 120cm in height and over 22kgs your cousin is breaking the law as well as not looking after her child's safety.
ETA also unless he is around 100cm he may not reach the minimum height requirements for a booster seat.
Edited by lsolaBella, 17 January 2013 - 12:00 PM.