No one can blame you for not thinking clearly with a newborn around! LOL!
I think what I'd probably do right now is slowly water those bottles down, so that at first she feels like she's getting milk but slowly she's getting fewer and fewer calories at night. Eventually you'll be down to all water and at that point hopeully she'll be making up those calories during the day.
Now, since she's 18 months old you can go cold turkey and simply say "You're a big girl now! You don't need milk at night. Mama will hug and cuddle you if you need it, but no more bottles." You could even have a big bottle throwing out ceremony (of course, you'll just put them somewhere else for baby) and say "DD1 is a big girl! She doesn't need these!" and then break out a batch of new sippy cups to celebrate. Perhaps even tell DD2 "Look at sissy! She's so big! She doesn't need ba-bas like you! You can't wait until you're that big, can you?" And then let's do LOTS of talk about all the great things big sissy's can do that babies can't. Even my son, who is an only child, LOVES to hear me talk about how babies can't do this or that but HE can. He even will say stuff like "If a baby tried to do this he'd just fall down!" LOL! Of course, your DD doesn't have those verbal skills yet but believe me talk like that will get the wheels turning. Much more distinguishing her for the BG things she does will help.
The cold turkey route will work, but you'll probably have 3 awful nights. If you go the gradual method, I wouldn't take more than 7 days to water down those bottles. Water them both down the same amt. That will cut her nighttime calorie intake really quickly.
Since the wakings are exactly the same time every night, they are habitual rather than true hunger. I realize she's downing those bottles so it *is* affecting her daytime caloire intake, but for her it really is a comforting habit more than anything. Since you do have a young baby right now, you can make daddy in charge of DD1's bottle withdrawal so that you don't get more sleep deprived than you already are! Or, you might bring in a granny for the weekend so that you can sleep during the day so that you have the strength at night to withstand any tantrums that might occur.
The key is to make dropping the bottle seem like a GOOD thing to her! I would act happy and excited about it -- like it just occurred to you, "Hey! DD1 is a big girl now! YAY! Let's throw a party!" In fact, you could even have a Big Girl Graduation Tea Party -- Invite grandma, aunt, cousins, friends, whatever -- just 2-3 people is fine -- and explain how she's a big girl now and doesn't need baby things anymore! You could even wrap sippy cups up in boxes and wrapping paper and let her open them like presents. maybe even a big girl stuffed animal to hug if she wakes in the night. Really celebrate the crap out of it, LOL! They believe what we tell them, so if *you* say that she's a BG with a bunch of fanfare, she'll really believe it. Even if you go the slow route, I think at the end of the 7 days I might throw a party like this (after a week of really talking up the advantages of being BG, of course!).
Don't underestimate the power of buttering toddlers up!
