I just checked a memorial I edited this morning, and see that a bunch of siblings have been added. When I click to edit family links, though, there is no place to edit siblings. I also manage the siblings, and know I didn't link them. I did link them each to their parents, though.
I'm guessing that siblings is now an automatic link when two memorials list the same parents? And half-siblings if only one parent matches. (What happens when only one parent is known? Do all the sibs show up as half-sibs?)