People who say "I seen" and "onder"

As in onder, oncomfortable, ongrateful, etc. What the fcuk? When does 'un' become 'on'?

One of my sisters says it the 'on' way. There is no half way house between 'un' ond 'on', so she obviously made a conscious decision to switch at some stage. When I hear her say words this way, I say "gee, is that the D4 accent rubbing off on you?" which I know drives her mad. She always takes the bait for this and gets annoyed, but yet she can't shrug the habit! Sometimes she'll be talking about something very serious and I'll still correct her! Priceless.

My other sister on the other hand says the "I seen". Now we're from a middle/upper class family, so she wasn't raised that way and she knows the correct way to say it. I think it's her way putting her 'working class hero' stamp on things - as if the say "I'm working class, I'm the salt of the Earth". Is it quite trendy to say it this way in some circles? It makes me cringe! Anyway, I corrected her recently (in front of the parents) saying "I saw". She flipped! saying "I'm a bit old to be correct now". I couldn't help but say "you're a bit old to need to be corrected. It's never too late to learn". She got even worse and left the room! I shouted "well you certainly won't learn anything with that attitude".

What do ye think of this? It'd be one thing if they were raised in those social backgrounds. But when people deliberately say a word wrong thinking that it's they're way of being unique! Hmmm.