Ok a couple of things I want to note here:
Firstly is sounds like he might be jealous of his younger sibling. Obviously if her behaviour isn’t as challenging you won’t be telling her off or enforcing consequences like you have done for him. This can easily be perceived as favouritism or being loved less by the child. However at 10 years old he surely has to understand that the reason he gets treated as such is to do with his own behaviour and that if his sister behaved in the same manner she would get the same punishments as he does.
If also sounds like he may also be envious of his younger sisters excitement for Christmas and believing in Santa Clause as a child is a big part of that. Perhaps he wishes to ruin it for her so that she is on equal terms with himself and he is also clever enough to realise he has gains to make by threatening you with telling her. I also don’t know how he himself came to know that Santa wasn’t real, but maybe he found that more traumatic that you realised, some children can be very upset by this revelation.
How I would handle it, firstly I would take away the power of his threat by not allowing him to use it as a bargaining chip to get what he wants. Remember, he may indeed get what he wants and still tell her anyway. Instead of reacting when he makes the threat, simply shrug your shoulders as say ok then. You can always think of things to say to your daughter should he choose to carry out his threat.
Secondly and with regards to managing his behaviour, long term consequences and punishments for him seem to just be escalating and inflaming the situation. I’m not saying you shouldn’t discipline him, or that he shouldn’t face consequences for his actions. Instead what I propose is that rather than bad behaviour and punishments being the focus, switch to focusing on the times that he is good, even if he only manages an hour. If he has been good for that hour, follow it up with lots of praise and love bombing. Make it so that the rewards for behaving well are so much more enticing and worth while, than the attention he receives by behaving badly. It will take time, you will really have to stick with it because it’s going to take a good while for him to flick the switch in his brain, and even then he may do extra bad things in order to test your responses. However over time and with lots and lots of positive reinforcement, he will see that it really is in his best interests to be good, and see for himself that the attention he gets for behaving well is better than the attention he gets for being bad, he will also find himself feeling on much more equal ground in terms of love and attention with his sibling and less likely to feel the need to compete.
For the times he is ill behaved do not allow yourself to anger (of course you maybe raging on the inside) instead, be calm but dispassionate. Firmly and calmly explain what he has done wrong, explain the consequence and then carry it out. Make sure you tell him that you love him, but you do not love his behaviour right now, and that when he is ready to be well behaved again he can come and apologise and he can go back to playing or whatever. Don’t repeatedly bring up his bad behaviour, once he has served his punishment let it go and go right back to reinforcing the good behaviour as soon as he behaving well. Also make punishments short and simple and something he can easily equate to his bad behaviour, e.g. no iPad/TV for an hour. When he apologises, thank him for saying sorry, tell him you love him and redirect his attention to something positive.
All of this will take some time, you’ve got to really stick to it to break the cycle but it can be done. Do be prepared for the fact his behaviour gets worse for a short time, this will just be him testing his new boundaries, it does not mean that what you are doing isn’t working, rather the opposite in fact.
I hope you manage to have a lovely Christmas 