Please help!

My kids are a bit older, but this problem applies to younger kids too I imagine, and I haven't been able to find any advice in parenting books/online.

My oldest is good about cleaning up after himself in the morning (breakfast, lunch-making), but the younger two (10 and 8) always leave a huge mess for me to clean up. I know they're capable of washing their own dishes and putting away the lunch making supplies, because they do it if I'm standing over them. But now that I have a new baby and am usually nursing in the morning it's harder to be there at the exact moment, plus I hate nagging.

I have tried setting a consequence for leaving a mess, but the problem is, they both blame each other. They both regularly make messes so it really could be either of them, or sometimes both. They've realized that if they blame each other, poor Mom gets stuck cleaning up the mess.

I've tried giving consequences to both of them if neither owns up, but that just encourages them not to do it in the first place. Each kid assumes the other won't clean up, and doesn't want to clean up AND get consequences, so neither of them bother. And my goal is to get them to actually wash their darn peanut-buttery knives and plates, not to punish them, so that doesn't feel good to me.

What do you all do when you can't figure out which sibling to hold accountable for something?