With my oldest son, who will be 7 on Sunday, he was a late potty trainer. I thought we would NEVER get him trained! But he was potty trained, finally, by his fourth birthday, and I always handed him a wipe and then checked, just to be sure for a while. It wasn't very long before he was demanding his privacy and if he needed help, was asking for daddy.
I would start giving your nephews a wipe when they go in, check behind them for a while, and when they do a really good job, give them a sticker on a chart. If they do a so-so job or forget to wipe, then no sticker. Once they get that chart all filled up, give them a special treat, but it has to be something that they really like but don't get to have all the time, or it won't work. Whatever his currency is, he'll start wiping on his own and being extra careful to get completely clean and before you know it, it'll be a habit. This is actually how we finally got my son potty trained, and I'm pretty sure it'll work with other things as well...from wiping after going to the bathroom to picking up their toys, to anything that they need to do on a regular basis that we moms have trouble getting them to do.
Good luck!