Hello Honey,
Welcome to Baby Whisperer. ((HUGS)) those night waking's (NW) are long!

Can I ask how long she has been sharing a room with DS

Do you think that sharing and him waking could be the whole problem, and she just isn't used to NW

Has she suffered from many NW in the past with a faster settling time

Also can you tell me if there are any other external factors that may be affecting sleep. Home life changes, for eg: New home, new nursery etc, developmental leaps

Anything you can think of

I'm guessing that before this she was managing okay on 11 to 12 hours

Most LO's of her age would need approx 12 hours after dropping the nap, but it does vary, I remember DS dropping from 13 hours to 11 for a while a few weeks after dropping the nap, and we just had to ride it out.
One other thing to consider is that this is a half birthday regression. Many LO's go through it on the half year and birthday's, if that is the case then it will just pass.
Will your OCD allow you to give her some early nights to try and help her cope for now

(I hear ya Sister! lol)
And finally can you rule out pain from an ear infection for eg.
Sorry this is all questions, tends to be the way with the first reply. Oh, and extra (HUGS) for the 18 month regression palava, it's one of the worst, hope he's through the worst soon

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x.