Jaiden had had over a month of waking up in the night due to teething (4 teeth in 4 weeks, it was a nightmare!)
Once he was done teething he just got in the habit of waking, and we (Menard and I) were so tired that we'd just bring him into bed with us and let him at it with the mummy milk. But I wasn't getting any much sleep, nor was Menard, and I was also getting terribly sore because it wasn't really that he wanted the milk, he just wanted something something to chew on. One night I dreamt that I was a sausage being chewed on!
So, we decided it was time to fight back. We'd try the controlled crying method - checking on him, making him comfortable and safe, but ultimately, if necessary, letting him cry it out rather than sticking him on the boob at the slightest whimper.
We arranged for Sophia to stay at grandparents for a few nights (we had her during the days!) put Jaiden's cot in Sophia's bedroom, and went for it. Wednesday night - he slept through*! Thursday night - he slept through! We concluded that we were waking him, and he was waking us, and that really, it might be best for all concerned if he stayed in that bedroom with Sophia once she was back with us. Menard did a whole heap of work moving things around and making it possible to have Soph's bed on one side and JJ's cot on the other.
On either the Thursday or Friday I was chatting with Sophia, explaining what was happening, and generally trying to big up the idea of the two of them sharing a room. She was all for it, which was a blessing! I was explaining that JJ might wake up in the night* but that it would be ok. She could sing 'Twinkle twinkle' to him (he responds well to that song when she sings it to him, it's very cute). She had a little practice, but then she came out with "STOP CRYING, BE QUIET JAIDEN, I'M SLEEPING". And I thought, yes, that's more likely to be an accurate representation of what will happen!
* When I said that he slept through, what I meant was that although he woke up once or twice, it was only to have a tiny cry or a little chat and then he self-settled back to sleep. When he'd been in our room, he knew we were there, and immediately wanted out.