as said, Ian's and thus presumably her parents were married in 1912 from when MMN was shown in the Birth Indices.
What leads you to believe from a photo, that Desiree and Ian were siblings? Is it marked up with names?
It could be an Indexing error or ommision - you state that Ian was her younger bro, so that homes in on 1912-1919/20 for her birth year.
What name was she shown as at death 2 years ago?
ADDED: This birth didnt show up in Ancestry that I saw, but it leaps off the page in FreeBMD ...
Births Dec 1914
Terry Dorothy D MMN=Hewitt Chelsea 1a 671
Chelsea was the District that Ian H N Terry is registered.
Possible marriage in Wandsworth, 1942 to a Colin F Speagell - who may have died in Grantham 1998