Actually, more often than not, missing siblings aren't there because one can't enter everything at once, and some other line is more important to the PM. As others indicated, you're more than welcome to join wikitree and fill in the missing profiles.
And yes, you can enter GEDCOms as a member which will add lots of people at once, but the quality of the product produced by a GEDCOM usually leaves much to be desired and will take much work to make right, which makes many people here just entering things by hand.
