How do you discipline an 18 year old that threatens to leave when you say no to something?
Curfews
Homework

The younger sibling 16 wants the same rules.

They are old enough to look after themselves.
The next time they are like this, call their bluff and pack their bag for them.
This should sober them up VERY quickly.

They MUST realise that this kind of behaviour will NOT be tolerated from an ADULT!

Why is you 18 year old still at home. Tell him he is of legal age and if he does not wish to follow your rules he can leave willingly or not so willingly

Let him leave, he will probably leave and come back in less than 2 weeks, this will give you a break from his snotty ways.
Also keep the rules in tact for the 16 year old, the older sibling will set an example for him when he comes back and the younger sibling will realize the grass is soooo much greener on your pasture.

How do you discipline an 18 year old that threatens to leave when you say no to something?
Curfews
Homework

The younger sibling 16 wants the same rules.
Let him go he will soon realize how good he has it at home and probably come crawling back ,Then you will be able to let him back under conditions.You have to show him who is boss because your youngest is watching.

My daughter is 18 and she has put us through some really ruff times.
But of course she is always sorry soon after, but if she threatened to leave I would say
Here's some luggage, in fact I have said to her when she was being a brat well you don't have to live here any more. People think that 18 is a magical number and they are sapossed to automatically grow up over night, NOT! It is proven that the brain is not completley
Done developing mentally until around 25. But regardless if he threatens to leave I would as said above call his bluff. Get him a set of luggage for his be day or christmass. LOL.

Yes, pack the bag, tell them my way or the highway, and let them decide.

My rules in my house and anyone who says different can go get there own house. That simple.

An 18 year old is a legal adult. If she doesn't want to abide by the rules and threatens to leave, then let her leave. As an adult she can choose not to follow the rules and choose to accept the consequences of not following them. Then make sure your 16 year-old knows that, in 2 years she'll have the same option.

I'll second everyone else and say, let him leave. Or, more importantly, tell him he has the option to live at your house and follow your rules, or he can find his own place and set his own rules. "If you make me do X, I'll leave" is only a threat as long as he thinks that you don't want him to leave. If you let him know that you are perfectly OK with that, it loses all power as a threat.

My 18 yr old son tried that once.. . so I packed his stuff for him. He looked at me like I was nuts and how could I just throw him out in the street with no place to go. I told him he'd better think twice before running his big mouth. Reminded him "adults" don't do that.
He hasn't done it since, and is looking for a place to live now.