My daughter was 2 and 3 months when my son was born,a nd she has always had sensitivity to ALL loud noises, so of course, his crying was a problem, and set her off for a while. We just started explaining why he might be crying and asking her to help do things to calm him ex: bounce the bouncy chair, singa song, give hima toy, help him put his binky back in, if all else fails, go in another room, so it is not so loud. We explained to her that when she cried too, it just made thebaby more scared, so he would cry more, so to try not to cry herself.
Now at 7 months and 2 and 3/4 years, it is going much better - just hink of this baby at daycare as "breaking in" your toddler for the new little one who will be coming to your house. Talk to her about it, 21 months is not too young to understand the stuff I said above.
Jessie