How to handle an overly emotional childs behavior?

my 5 yr old daughter is very emotional. she cries alot at everything. she once cried bc her older brother picked a flower she liked out of the ground and she was crying like she just got hurt really bad or something like screaming crying and i was like what happend and she replied he picked my flower..and i was like omgosh u don't scream bloody murder over a flower i really thought something was wrong or she was hurt..i of coarse took her sadness into consideration but like really?!? and when she gets in trouble the tears roll also when sibling r mean her older brother and her r really close in age and they used to get along perfectly but as soon as they both started school they started to always be in competition..she always plays the "no one loves me card" or she will say bad things about herself and really makes me upset to hear her say these things bc i try so hard for them to know they r loved and smart and give positive remarks. she has very low self esteem sometimes not all the time i just don't know how to deal with her crying she doesn't just sniffle she cries bloody murder everytime..most of the time i send her to her room to calm down then we can talk when she is calm which can take up to an hr then she falls asleep. her brother doesn't help he is always teasing n bothering her like brothers do her baby sister is very caring and often tries to comfort her when she cries telling her its ok don't cry i'll be ur best friend :) and she is only 2..my son is totally different he is the type to get angry at everything from when he gets bad grades which arent but he is a perfectionist and beats himself up if he does something wrong ...and my 2 yr old daughter is well a baby but very smart and seems to handle her emotions appropriately but i want to handle the 5yr olds behavior b4 the 2yr old starts to behave like her..so what should i do to deal with my overly emotional child..she is very materialistic for some reason and only behaves and says i love u most when she gets stuff which isn't very often bc i don't reward them for bad behavior they have to earn it. please help if u can..only positive feedback..