@WimpoleHat
Alyssa will be smug, I bet - she's basically won. She gets her mum to herself and everything her own way, as at home.
I don’t know - for once, there have been some consequences for her actions. She won’t have done what it was she thought she was going to do with the OP’s family. She won’t have had OP’s daughter for company yesterday. She’s seen the (natural) consequence of her behaviour, which is that it makes others upset and not want to spend time with her. So possibly a valuable lesson at some level?
We can but hope, for Alyssa's sake!
I agree that this has been a very interesting thread. Like a previous poster, it has made me sit up a bit straighter wrt both of my children 
DS1 is a boundary pusher, like his father. This wasn't such a problem when he was smaller, but DH did indulge him and I seethed at being forever cast as bad cop. DS1 is now 10yo and has crossed the perception line from 'funny kid acts up' to 'entitled dick who needs a telling off' in the eyes of the world in general, not least DH. So DS1 is now struggling to cope with DH's firmness towards him (although DH maintains he has always been strict
), and clings a bit more to me, the one who has least been consistent in being a bitch firm throughout. It is not always easy though! He and DS2 have a very well-behaved friend visiting atm, and DS1 has acted up - the consequence is no treat breakfast for DS1. DS1 is being (by turns) remorseful, reproachful, sulky and outright rude about this, but the friend has been quietly nodding along with a 'Well that's reasonable mate tbh' face whenever I rebut DS1's complaints. My confidence is buoyed by his friend's tacit approval of my fairness 
As for DS2, he is under assessment for ASD and is not always easy to manage. The trick with him is to assess what he really really can't do, and what he could do but doesn't want to. The former, he gets given latitude; the latter, absolutely no fucking way. To his credit, he takes this pretty well. It's a process...