Some hammies are more sensitive than others, just like people. I'd try to do what Krill says and move them away from each other. You might have to have Bertie have playtime in the bathtub with her own toys so she has no scent of Willow around...at least until you can be sure that it really
is Willow's scent that's upsetting her. You can also just have her playtime on the bed or couch with you while observing if her nervousness stays the same. Hope this helps.