My parents could have let me go to school a year earlier (I was born on what was back then the cut off date) but they didn't and I resented them for it for years. I was always the eldest in my class and sometimes year level and I hated it.
If your daughter is so excited about going to school and is ready for it then probably best to let her go. I am sure she will still bond well with her younger brother and you guys can still do some of your weekly activities etc outside of school hours.