Agree that you must sit them down. First prepare. Take photos if the kitchen bomb site! Make a step by step photo story of how you got it all tidy and clean. Use this as a friendly way to show what they must do.
I have a 14yo and 16yo too and they are not too bad most of the time (though the first 2 weeks of term they were tired, had colds, and were very busy so the mess started building up. I left it for a week before making noises about specific things not done.)
I think what works for the 16yo is to raise the point that she is going to be away at university in a couple of years, she needs to be well organised and tidy and keeping up standards of cleanliness. She would expect that of others, and could be given notice on any rented accommodation where others complain about bad habits. Home is where to learn to live in a shared house amicably, even when you don't feel like it.
My 14yo needs a different motivation. She likes to bake cookies as gifts now and again, so I insist she cleans up after herself on a daily basis so that I know she won't leave a bigger mess when she bakes.
It is easier to work with them individually, and that got them into good habits. Then there was a time when if they used the kitchen together they just blamed each other for the mess and refused to help each other. I threatened to stop giving them lifts to clubs, having their friends over, letting them watch TV and took their phones off them until they could get it done. It took a good many goes and reminders that it is important to get along, etc.
We have frequent visitors to the house, so that helps too.
(Did anyone else expect this to be a thread about how the OP could get up on the roof of their house?
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