I'm sorry that your daughter is having such a tough time which is therefore impacting you. Typically when I've seen this behavior in young children something has happened that they perceive as big even if we as adults haven't given it much thought. These incidents that triger increased sensitvity can be either positive or negative. I would do I close investigation of changes in her life. I noticed that you didn't mention her father's response to this. What is his relationship with her like, have there been any changes therein? What are her thoughts about her new teacher and the kids in her new class? Is your younger daughter now going to the same school as her? Has your stress at work or elsewhere increased? What significant changes (good or bad) happened this summer, was she is camp or away with family? How does your oldest daughter perceive your relationship with your youngest.....as they get older have the ways you all interact together and seperately changed significantly.
It may be helpful to talk to her about the changes and then brainstorm other coping skills besides crying. Ask her to draw, write, collage, sing about, or act out her feelings. Also help her to discuss the advantages vs. disadvantages of crying (i.e. extra attention vs. frustration from her sister). I would also conference with her teacher about how she copes with frustrations and disappointments in class. I hope these reflections help. Please ask me any other questions so that we can move towards a resolution of your daughter's intense sensitivity right now. Although it may just be a phase, working through it as oppossed to ignoring it is still important.
Hello, have you been able to try any interventions with your daughter. How is she doing this weekend?