Did the girls share a room before the transition to big girl beds? If not, this is all new to them so it's going to take longer. You put them down, and read them a book, tuck them in and say 'Good Night!', lights out. Give them 30 minutes of talk time, this is their bonding, venting about the day, quiet time. You can even tell them that they have a little bit of time for themselves, but they have to stay in their beds and use indoor voices, then it's quiet time. You'll be in to check.
You can also use the consequences of good behavior. Create a chart with their names and dates on it. Let them know that the first one to fall asleep gets a gold star/sticker on their chart. And the first one to get 10 stars gets 'a special treat', you decide it could be a one on one trip with a parent to the grocery store, a new book, whatever...
if they're both asleep when yo come in to check, they each get two stars. Keep this going for at least two months. You'll need the routine to keep going when the new baby brother arrives.
Congrats on your impending arrival!! You have two great little helpers who will be anxious to help in every way. I strongly, and I do mean strongly suggest that you get them each a baby doll to care for, one that they can feed, dress, diaper, rock and be Mommy to, so they leave the care and feeding of the new brother to you. I did this for my daughter with each of her brothers and for my oldest son when our youngest arrived. It helps so much!!!