You are right -there is very little information on the daughters. I have been collecting fragments of information for years and I will soon be updating the page on Lightoller's family (
Titanic's Officers - RMS Titanic - Second Officer C.H.Lightoller) and will let you know if I find anything. One aspect to consider is that Sylvia outlived Herbert by 17 years.
I do have a wonderful photograph I'll be adding soon of Herbert and the whole family in swimming costumes in Port Victoria -the youngest child Brian sits in front, with sisters Doreen and Mavis to their left. But they are all huddled very closely together which to me at least indicates they were a close family. And in another photograph of the two girls and Trevor in 1933, this time the girls are grouped close to Sylvia seated at a table, and Herbert looking over them. It is dangerous to judge via photographs, but the sense is that they were indeed a close family. By the way, I know social media is not for everyone but a lot of these photographs and more are frequently posted in the Titanic officers Facebook group long before the website is updated (
Titanic's Officers (Official)) if you are interested.