Bath from the sounds of it maybe this was inevitable whether you spoke ill of your mother in front of your DD or not. From my point of view any young person wouldn't like being told not to kiss their younger brother and would definately get the feeling that your mum didn't want them to hold the baby for too long or unsupervised. This alone would make any child defiant I think. I know that I was 13 when my youngest brother was born and I would have reacted the same way if anyone had treated me that way with him. So maybe its not all your fault IYKWIM. I know alot of kids these days (regardless of how they're bought up) like to be shown respect from elders as well as vice versa if that makes sense. ie In the past your eldes were always shown respect irrelevant of their actions etc, these days kids won't hesitate to say 'hang on a second that's not right for you to be doing/saying that to me and I don't like it regardless of the fact you're older"...

hmmmm does that all make sense? It may not even relay back to your situation at all but that's just another take on it. And I haven't given you any advice at all LOL. I hope it all works out for you though!