I am speaking from experience...a very similar thing happened with us. Generally, my advice would be is to try and keep as many things unchanged in her life as possible. For example, if she went to nursery before her baby brother was born, she should go as she went before...and the same applies to bedtimes, by which I mean the bedtime routine--hours, stories, whatever it was before the baby.
Regarding crying, I am not a fan, of leaving children to cry it out, esp. such young children trying to cope with the pain and fear the newcomer may cause. That is actually a very understandable reaction. However, routine has to stay the way it was before, and as some others have suggested before (this is what eventually I did), go in every few minutes, reassure her that everything is ok, express your love for her in any way you feel, but be firm, don't let it go on for more than half a minute, and then walk out confidently, and calmly...you may have to do this hundreds of times, in the beginning, but in the end, I think this is what helps. You recognise her pain and put through the message that you care, but by your calm and confident attitude you also show she has nothing to be afraid of.
Main point is, that she should not be lead to think that anything has changed in your relationship with her....if you go in and sleep with her, either way, she might get the impression there is a good reason she is afraid and upset.