Do Seamus and William know they're cousins?
They do, yes!
Although it’s mainly because of Stephen the year before. The Kirkland Bros didn’t even know they had cousins coming to Hogwarts, and Kirkland isn’t quite so weird a last name as Malfoy so they kind of ignored it when a Stephen Kirkland joined the school. If it wasn’t for the fact that Steven is extremely energetic and loud and doesn’t at all mind approaching older students and asking them if they knew his dad, they’d still never know.
At first the Kirkland Bros were kind of hesitant to accept they had cousin - they’d only just got used to having brothers - but Stephen is tenacious and in the end they got quite fond of him. As a Gryffindor he gets along particularly well with Tadgh and Fingal, but weirdly enough it’s Arthur that’s his favourite.
So Seamus and William know they’re cousins, but they barely know each other so they forget most of the time to be honest until someone brings it up and they’re like ‘oh yeah’.