It does rather sound as if most of what you wish to find may be too recent to be accessible or posted on here, in that case. Censuses only available up to 1911 as others have said. It'll cost you to see 1939 registers, too - and it seems a bit of "If you know it already, they can help you check it out" with that register.
If you know the area of the country you are seeking family in, then a plea via the local newspapers may produce some results. If you do know the area, and dates/names of these "dead siblings", then there may be people still who were at School with them who may be able to help, in the local areas. Would baptismal records be a possible help? Most R.C. baptised children seemed to attend R.C. schools, for example.
I think you may need to be more specific, in order to be helped. But there are some fantastic "finders-out" on here.
