how old is the oldest brother? why does he seem like he's interested in an 11 year old??

Sienna is 12 & to join the military one should be about 17 so Aciel is probably around that age (i forgot). Anyways nothing happens until they're older so you can rest assure

I read spoilers that in her past life he comes to accept that he loves her just before her suicide so maybe when she's way past 20. Then the romance in this life should be when she turns 20 as well.

It's popular trope in otome genre to write FL ML meeting early & being friendly but none are brave enough to write a pedo story. So it's like that type of story.