J.,
Wow! 2 in there at once! My son is 2 and occasionally lands in mine, and that's more than I need.
You mention your daughter is almost 5. Start building up the move to her big girl bed before you actually make the transition. Take her out to pick out new sheets and a blanket, something she's really excited about. Talk up how she's going to sleep in there once she's 5 and "big", that way if your son doesn't follow, she can see that he's still "little" and that's why he "has" to sleep with Mom.
Put her new bed together a bit before she's going to sleep in it, so she can see how nice it looks, play on it or put her dolls and stuffed animals in. Let her see that it's not threatening. Try putting a baby monitor in there, and letting her test it out. Let her whisper Mommy in her bed, and see that you can hear her if she needs you.
If this doesn't work, you can try a sticker chart, where she gets something special when she makes it a certain amount of nights in a row in her big girl bed.
If all else fails, you can go the hard way. Put her in the bed. When she gets out, put her back. It's going to be rough for a few nights, but then she'll get it. If you go this way, you've got to stick to your guns. One time you let it slide, and she'll never stop trying.