I’ve been researching my family and managed to find the marriage certificate of my great great grandparents - but on the same page of the book was a really weird record. A marriage between two siblings. Same surname, same father, the witness appears to be their mother. They could potentially be half siblings, but that seems really unlikely. I thought it could potentially have been the case of a guy marrying his brothers widow, but her status is as a spinster - not a widow. Was this legal then? I haven’t been able to find anything about it online. Was it something that happened often, or like maybe a religious thing?
Here’s a pic of the record - https://imgur.com/a/jtjVJCo
No you definitely couldn't marry a sibling.
Looking at the certificate, the husband comes from Middlesex and the wife from Lincoln, so I doubt they're siblings.
I'm wondering could they be cousins or something? The chances are they are related to have the same surname. William was a very common first name and families often used the same names within familes.
I would try looking up the familes in the 2 different counties and see if you can find a connection between them.