Sorry, Poppy :(
DS slept like a dream until DD was born. In fact, he stopped sleeping the day we brought her home from hospital. He was exactly 2 and a half years old when she was born, and we'd done a great job building him up to it, explaining about the new baby and getting him to help getting cot and clothes ready.
When DS was younger he would have slept for anyone, anywhere and settled quickly and easily.
Now myself or DH spend up to two hours a night sitting on his floor, sitting on the hall floor outside his door, doing the Super-Nanny thing of putting him back to bed etc. All this while I breastfeed DD and try to get her down. Then once he's in bed asleep he'll wake up anytime from around midnight and be a raving banshee shouting and wailing. For a quiet life DH has gone in after he wakes up the 2nd time (usually about 2am) and just sleeps on his floor on a blow-up mattress.
I used to be so proud of his good sleeping habits. Thought I was a great mum. I now realise it was all pot luck, and that has now blown up in my face.
Sorry to be such a misery, but it has made me miserable. DD has been a crap sleeper and crap napper from the very beginning, and with DS as he is too both DH and I are close to breaking point sometimes.
But, for all my misery, there are many many people I know who have two kids who sleep perfectly and all is like a fairytale. Hoping that happens for you, Poppy!