My vote, siblings.
I see this as a similar situation to a friend of mine. She and her husband could not have children naturally. Through an interesting turn of events they adopted a little girl privately through family friends and then within weeks (months?) they got a call that they could adopt a little boy. So similar in that the moms were pregnant around the same time and 2 different moms. The kids are siblings.
Interestingly, my friend said she got the funniest looks for the 1st year or 2. Double takes because they were far enough apart that it was clear they weren't twins but not far apart enough to be 2 sequential pregnancies. (also the little girl has dark hair and quite tanned skin while the little boy is the palest redhead). By the time they were pre-school, the little girl was very tall for her age so they looked at least a year apart, so far fewer explanations to strangers after that. :-)