so i went through the same thing. my daughters are 21 months apart. this is what i did and it worked. i changed the crib to a toddler bed so she could get used to it. she LOVED IT. it was small she could get in and out, and it had the guard, so there was only a little space she could get in and out. to keep her in it, the first few nights were tough. we stood outside her door, took turns and told her to get back in bed. sometimes for like an hour or more. but she got the point. she didnt see us so she didnt know we were there. she got the point.
then, we got her an elmo bed spread, her favorite, and put it on the other bed in teh other room. she kept going in to look at it. and then i finally said, about two weeks before the baby came, you wanna sleep in a big girl bed now with elmo? LOVED IT. so we have the twin bed without the frame, but with the box spring. worked like a champ. we brought her things int eh room right away and hung them, so she got to help to make it HER Room. she still stays in bed, most of the time. then we changed the toddler bed back to the crib, and then it was a baby bed again for her little sister. hope it helps
good luck