Our two girls(just turned 2, and 4.5) began sharing a room 6/3/15. The first week we had small issues with the youngest getting out of her bed and climbing into her sister's bed, mostly early in the morning, 6am vs their normal wake up time 7am, I wasn't super concerned with this because they are staying in the room, just affecting their sleep pattern.
Then, the youngest started climbing into bed with big sister at bedtime, so I have been consistently "catching her in the act" on this, returning her to her bed with a small pop on the bottom, "stay in your bed" message at night and in the morning, with the help of big sister, they may only play on the floor together, no getting into the bed.
Now the youngest has been waking for a week daily between 3-4:30 am, getting in bed with her sister, does this so quietly that I don't hear her, until big sister knocks on our bedroom door to tell me the youngest is in her bed again.
I don't know what to do, the youngest does take a 1.5 hour nap, she was a fantastic crib sleeper, I would generally have to wake her up in the morning and extremely rare nighttime wakings. My oldest went to visit grandpa for a week and while in the room alone, we had no issues with the youngest.
My oldest falls asleep within 5 mins of her 7 pm bedtime, the youngest with the same bed time takes 45 mins to an hour on average, I usually have to go in 1-2x to make her get back in bed.
We need them to share a space because we are expecting in August, my husband is deployed and I am sadly getting more and more worked up over the situation.