How do people look for siblings if the birth is after the cut off date?
With difficulty, until you get to 1929 of course.
If you are actually in the SP centre you can look at any certificates you like. From 1974, the maiden name of the mother of the deceased is indexed in the in-house index, unless the birth was illegitimate. So if I am looking for siblings born between the 1911 census and 1928, I usually start with the death index.
Once I have found one or two children of the couple involved, I have their exact dates of birth, so I can look at their birth certificates, and note which district they were born in. Then search all births with that surname in that district to see if there were any more.
Sometimes you can be faced with a long list, but you can home in on ones who have likely-looking middle names - maybe named after grandparents or aunts or uncles.
It dosn't always work, of course. Sometimes a surname is just too common to go through all the births of that name. Sometimes people moved around a lot, so you can easily miss a birth if it's in a different district. This is especially so if they lived in one of the big cities which had lots of registration districts.
