This thread is bloody ridiculous 
OP, there is nothing wrong with wanting your 18 month old toddler to sleep in her own room. There is nothing jealous or resentful about that, ignore the preachers on here who think you should be co sleeping until your DD decides she doesn't want to. You're about to have another baby so the sleep issues need to be resolved before then, otherwise everyone is going to be sleep deprived and miserable. Co sleeping is no longer working, and that's fine.
You said your DD is sharing a room with her DS, is there any way you can get a cot for her rather than a bed? The bed might feel big and exposed which is why she can't settle. My 22 month toddler hates the bed but loves his cot and a sleeping bag. If the cot isn't possible is there something you can do to make her feel more secure? Sides on the bed or something? There are sleeping bags you can get which have legs in them.
I'd start by getting DH to sleep on the floor in DDs room until she's happy to be left, it will happen eventually and as long as she isn't distressed it will be the best thing for her.
Then have a think about what you want your night times to look like once the baby arrives. Clearly, if DD has been moved out then starting to co sleep with the baby doesn't seem very fair and is likely to make DD refuse her bed, it's going to be tough on her anyway. And if you do co sleep with the baby then you are probably going to end up in the same situation 18 months down the line.
FWIW mine were both in a Moses basket in my room until 5 months, when they'd outgrown the basket they went into cots in their own rooms. When they woke in the night we went to them, avoided taking them out of their rooms where possible and they weren't distressed by this. They've both slept well since birth anyway. Now my eldest gets out of bed on a morning and comes in with us which is lovely but no one gets back to sleep. There is no way I'd let him in my bed overnight, we'd all have rubbish sleep and that would affect the next day.