They are pretty consistant with Shourei and Ryuuki. Shourei is 16 when the story starts and 17 (explicity stated) one year later. Ryuuki is stated to be 19 when the story starts.
Now, Ryuuki is the youngest of six (or seven?) half brothers and Seien was the second eldest.
Seien was banished when Ryuuki was 6 (explicitly stated).
Seiran was taken in by Shourei's family 12 years before the story starts. That puts Shourei at about 4, which fits the scenes shown. BUT, Seiran also states that she liked a certain kind of tea when she was 1 or 2 years old, which he should NOT have been aware of.
Seiran is stated to be 21 when Ryuuki is 19.... and that's where I get really confused. Because he's supposed to have gotten swords from his father at the age of 12 and was banished soon after. Add 13 years to that (1 year in Sa province, then 12 years with Shourei's family) and you get 25. Not to mention that 19 to 21 is only two years, meaning that when Ryuuki was 6 Seiran would have been only 8. (but 6 plus 13 DOES come to 19)
25 actually makes more sense when considered with the flashback scenes to 8 years before the story starts. That would put him at 17 to Shourei's 8 and fits what we are shown. 13 didn't fit.
I supposed it could be translation issues, but, sheesh.