Incest is when siblings have intimate relationship but this one are not thought. They just shared a lover unrelated to them and not being lovers themselves so it's not incest.
You can search it up if you don't believe me though.
two siblings sharing one lover is not incest, as incest specifically refers to sexual relations between close, blood-related relatives, such as siblings, parents, or other family members. The relationship in question is between two siblings and a third, unrelated individual; there is no blood relation between the two siblings themselves and their shared lover that would constitute incest.

ARE WE FINALLY DONE WITH THE INCEST???
the story would've be better if the 2 tops were just rlly close friends like