Not sure I can offer much help, but just to let you know we had a similar phase a few weeks ago but our DD seems to be doing better now. She is now 27 months and also has an 8 month old baby sister, so similar situation to yours. Our nightwakings were prob related to her newfound imagination and fears... She called for me and didnt want to be left a alone. She sounded scared and also asked things like whats under the bed? She kept saying "somethings coming", so it really seemed loke genuine fear. I took her in with us a couple of nights, then also spent a few nights curled up on her armchair in her room with a blanket, because I just couldnt keep going back in so frequently. I talked to her every time she woke, really quickly before she got worked up just tellong her "Im here". We just continued like that, reassuring and saying that we are all here etc etc. She seemed to get over it by herselft... Although she still needs someone with her to fall asleep at BT.