Is she keeping DD1 awake? Is that the problem?
DS used to be like that. When he was younger I couldn't leave teh room at all, but as he approached 2 I could, so that was easier. It took 1-2 hours for him to go to sleep some nights, but not having to spend all night there made it easier. Maybe this is just her process for getting to sleep at the moment?
No actual advice, sorry. DS just seemed to get over it - still takes him a while to get to sleep, bu thtere's less noise now