C.,
I would start by sitting down with your daughter during the daytime and talking to her. Tell her it is time to come up with a plan together for her to sleep in her own bed. She is old enough to convey her feelings. Try to find out why she does it. Is she scared to be alone? Is she in need of a security item (stuffed animal, nightlight, etc.) Is the monster from the scary TV show in the closet? Does she need more daytime mommy one on one without baby brother? Does she need a more established and consistant bedtime routine?
Step 2 is to tell her that you will work with her to make this a comfortable transition, but she is no longer sleeping in your bed. (Do NOT say this until you are ready to fight the battle. If you say it, then give in to her demands at 2 am just to get some peace when she has been fighting you for several hours, you will have to start over and she will be that much more determined to force her way into your bed because she knows you will cave in.
Step 3 is the plan. This will depend on your child. You may need to sit in the corner of her bedroom reading for a week while she falls asleep. You may need to lay down in her bed with her for a few nights. You may need soft music and some lights. Get her input. What does she need? Follow through.
Step 4 is the reward. Have some sort of a reward system for staying in bed. You can start with a reward just for staying in bed all night, then move to staying all night without a fight. Again, get her input. 1 star means a piece of gum. 5 stars means she picks dinner. 10 starts means a movie night. 20 stars means Pump It Up. What does she like? *** It is very important that she doesn't get a choice of mom's bed or reward. It's just reward for working towards a goal, not a choice. ***
Step 5 is the consequence. She needs to know what will happen if she comes in your room. You will put her back in her bed. It's not the time to discuss rewards, just walk her back in and tell her to stay there. You may need to get firm and lock your bedroom door or hers so she can't get in your room. Make sure she knows this beforehand when you are making a plan so there aren't any surprises. (If you have to lock her door, use a baby door knob cover or take the doorknob off and flip it around so it locks from the outside.)
This all needs to happen in one day, so get ready ahead of time. Make sure you and your husband are on the same page. You can't undermine each other on this one. It will get worse before it gets better. Prepare for a month before she gets over it. It may not take that long. Stand firm and do not give in.
Good luck,
S.