Help with sleep

So I have a 2 1/2 year old who was very easy to sleep train and till this day a greatttt sleepers. I also have a 15 month old who i cant even get into her crib anymore. She will hold so tight onto me as soon as I move she screams!!! I have tried letting her cry it out even and only occasionally works on naps not night time. She will scream and yes scream not cry till she pukes. I have baby number 3 on the way due first week of July and I am very worried how hard it will be if i dont get my 15 month old sleeping on her own! Any help or suggestions??? Thanks