3 1/2 and 4 1/2 year olds still in diapers?

Thats a bit late to potty train, kids should start training by 2, my 6 year old brother was trained at 18 months but my 11 year old brother wouldnt potty train either and is still in diapers so i know what its like. I would just take away the diapers completely, that way, they would have to potty train, if they like it or not