Hi MM,
NW are the worst and I can imagine that you are feeling pretty exhausted now. I understand to an extent how you feel WRT thinking she is old enough to 'know better' I have BT myself in those thoughts. However I have tried to remind myself that as we see our LO's grow from a baby to a toddler, it can be easy to believe they can 'help' these things, but the reality of it is she is little more than a baby herself at just 2.5. It does sound like there is a lot of things in the mix for her, as well as for you, 3 LO's of the ages yours are is a lot for any Mammy to deal with, especially with them all needing different things, and being at different developmental stages. I do think it probably has a lot to do with the new baby coming along and also that every time another child joins the family the dynamic changes and the attention you are able to give becomes less. Of course she will benefit greatly as they all grow up together

but for now it's hard. I have only 1 DS, so please don't think I am proclaiming to be an expert here, I only know how hard 1 child's developmental issues can be, so I sympathise.
So I wonder if re-assurance and lots of praise and love from Mammy during the waking hours could be the answer. Remind her she is the big Sister now, and how well she is doing. Tell her you know how well she can sleep, and maybe start a reward jar, chart if she STTN. As hard as it is, I do believe you need to go to her when she wakes, of course you can keep the interaction to a minimum, just tell her "It's still sleepy time Darling'" and give her a kiss, which will hopefully be enough to re-assure her and keep the disruption to a minimum for the others. Also, have you heard about 'sleep talk'

I have used it with great success with DS as have many other BW

You go into her sleeping between 1 and 2 hours after going over, and use a 'speech' to help her with what she is struggling with. If you would like to know more, I could write you a speech and you could give it a try. If she is struggling with the changes, it could really help, and it only takes a minute each night

I know that for many of us 'our ideals' when we become a parent can become blurred, and we go down roads that we would rather not need to go down, but tiredness and frustration blur our vision, it is hard, really hard. I post on here so much for all kinds of issues, and very often it takes another BW to help me to see things clearly, because I can't see the wood for the trees. I always try and take it on the chin, even when it is something I don't want to hear. I think about Tracy's ethos and what she would have advised me to do, because this site is her legacy and as moderators we all do that in order to honour her memory, we simply can't and don't advise on any methods we know she would have disagreed with.
Here is the link for the sleep talk thread on here, you could maybe have a look through and see if it is something you would like to try.
((HUGS)) I'm sure we can help you through this.
Vicki.x.
Anyone want to talk about Sleep Talk?