I have an old-style Family Tree Maker database (.FTW) which contains people who are siblings, but for whom the parents are not known. In FTM, I can click on "parents of" one of the siblings, at which point a record is displayed with no names for the mother or father, but the three siblings are displayed under "children".
When I imported this database into TMG directly from the FTW file, all three siblings are listed as individuals in the index, but there is no sign I can find of a relationship linkage between them. The "siblings" window shows no siblings for any of them.