Hi Maggi,
I'm sure you've heard it all by now. I have two boys, age 51/2 and 2. My younger, well if he had been the first, he'd have been the last! As I'm sure you know, babies go through developmental phases and it's around this age that they wake up and work things out. Both of them did the same thing around this time. It seems that their baby clock is set to go off, regardless of what you plan or do and you're not alone. It's a phase that all babies go through and unfortunately your need for sleep makes you compromise. I did too.
I'm not an expert so I won't preach. But that said, you need to be willing (on a weekend--start Friday night) to let him learn how to settle himself down.
Try this,once the sleeping twin is asleep, is it possible to move him into big brothers room? Turn on a fan for background noise so if he hears his brother he'll stay asleep. If possible keep him in his own crib.
Now "wake-up" baby won't bother his brothers. Hopefully.
What's important to remember, he's safe in his crib and tears/tantrums do give way to sleep/exhaustion. He's flexing his "mommy" muscle and you need to be strong. Every mom and dad goes through this test, it's a learning phase and babies are good at guilt!
Be warned! He will not be happy, but if you let him go through is emotions and settle himself down by Sunday night, you should have a better handle on this. What's happening is he's getting into a routine and your his 2am playmate.
Hear me when I say, YOU ARE NOT A BAD MOM/DAD, if he cries. I know it's tough and it's hard work, but you are the parent and your other children need you too. Stand tough, both of you and you'll see results quickly. If it gets too much turn off the monitor. He's safe, you know that. Don't put your emotions in his place. Step back and you'll see a little genius at work. don't be fooled. You can do this!
Best of luck- kate