Hi T.,
We had three babies within 3 years and 3 months. Our oldest was still in a diaper at night, when our third was born. (Which meant we had 3 in diapers at one time!!) He only needed the diaper because he didn't "go #2" on the potty. I think stress and fear probably was his reason. He'd struggle around in his sleep, at night, moaning and crying with a tummy ache, because he hadn't poo-poo'd in several days! There really wasn't anything we could do to "make" him go during the day. I just tried to take the stress he was feeling away, by de-emphasizing the whole issue. When he did, eventually, poo-poo in the potty, we praised him, but also didn't overdue the issue. It wasn't long after our third, that he was out of the night diaper...I know you don't want to wait that long, and probably won't have to, but every child is different, as you know.
I also believe having another baby in diapers can slow down the big brother or sister, as far as potty-training goes, in some cases.
As encouragement, let me just say that our son is nearing 20 years old and DOES "go #2" on the potty! :) I remember our pediatrician telling me not to worry about it, that our son WOULD conquer potty-training, and would not be wearing a diaper to kindergarten! I didn't really appreciate that advice, at the time, but it is true.
Anyway, I hope this helps, at least in knowing that you are not the only one whose child has this nighttime-diaper issue.
God bless, K.