Elizabeth E. Wein's The Winter Prince has some major brother-brother incest undertones, although there's only one expression of it and it's...dubious? Seriously not healthy, but it is Mordred we're talking about (she also wrote a short story exploring his incestuous relationship with his mother, which is very very dubiously consensual). Medraut and his sister Goewin have some odd tension, too.

Arthurian stories are pretty loaded with (at least initially) consensual brother-sister (or in some versions, nephew-aunt) incest, although that may not be what you're looking for.