I found a record that gives me the name a sibling and the birthplace of the parents, but no name of the parents. Is there a way to add a sibling without having parents?
No, there has to be at least one parent added in order to connect siblings. When I encountered this problem I added a father with the same last name as the siblings. Of course they could both have been illegitimate and taken their surname from their mother, but the circumstances aren't too important when the idea is merely to link the siblings on Family Tree. Obviously you can add a Note to the profile of the unknown parent to explain what you have done.
I wonder under what circumstances you found the siblings connection? In my case it was an individual naming his brother (by relationship) in his will. This was an early 18th century document and the parish records did not go back further enough to include the siblings' baptisms and hence their parentage.
There are only two types of relationship in the Tree: couple and parent-child. Everything else must be (and luckily can be) expressed in terms of these.
I have several cases where I know a sibling's name and information, but nothing about the parents. They're from funeral notices, which generally list the still-living relatives of the deceased. If the one sibling was born in a time and place where birth records are unavailable and marriage records are uninformative, then the same is generally true of the other siblings, but it's still worth it to track the side branch (from my point of view), because it at least doubles the chance of finding that "breakthrough" record, eventually. And in the meantime, as Paul says, a placeholder parent (father) with just a surname will allow you to connect the siblings.
I know that my 2nd GGF had a brother and a sister from various records, but, to date, I've found nothing on the names of my 3rd greats. So, a placeholder with just the surname is what I've done.
good idea thank you