YA Fantasy romance novel about a girl whose brothers go off to war once they reach a certain age

Question

I'm not even sure if this book even exists or I have dreamed it. It's about a girl, I remember she had an older brother that went somewhere (war I think) once he reached a certain age. The oldest one died, and the birthday of the the middle one was just around the corner, so he was the next one to go.

I do remember they were really poor and lived in a somewhat caste society or there is some hierarchy. One day mom sent her into the city to do some work (tailoring?) but she had also some plans there so she tried to hide from guards. When she started to get out of the city some rebels made chaos, she almost got caught.

Answer 1

As the asker said, it is Red Queen by Victoria Aveyard. Per Wikipedia, the first part of a summary seems to match (emphasis mine):

Mare Barrow is a Red living with her parents and a younger sister, Gisa. Her three older brothers, Bree, Tramy, and Shade, serve in the front line of a war fought between the northern Kingdom the Lakelands and the Barrow's homeland, the Kingdom of Norta. Norta is currently ruled by King Tiberius Calore VI, one of many "Silver" citizens, whose silver blood and supernatural powers allow them to rule over the more numerous yet powerless red blooded population. Mare is jealous of Gisa because her skills in sewing earned her a job working for Silvers, and Reds who have jobs don't have to conscript in the war. When Mare learns that Kilorn Warren, her best friend, will be conscripted, she plans an escape and meets with a colleague who directs her to Farley, a captain of the Scarlet Guard, insurgents composed of Reds who want to bring equality between their people and the Silvers. Farley demands a sum of money in exchange for Kilorn's escape.