>>527554159
Twins can be fun and often becomes a source for speculation from the audience, but older brother is usually used as a stand in for the reader, as he commands authority and usually these romances are flat.
In Coffin it's used more purposefully, as Andrew being the older brother has a huge role on just about everything(them having sexual awakening at roughly the same time due to the age difference, him having to raise her and becoming a doormat, etc).