Board Thread:Supergirl discussions/@comment-9346948-20171128012113/@comment-1410838-20171203192433

It would really depend on what state Central City is in and that states legal definition of incest (in the comics, Central City is in Missouri, so ... yeah, the South and all). Some states don't classify such activity between foster, step, and adopted siblings as incest and thus, as long as they're over 18 and not in a situation where they're dependent on the family, they can bonk themselves silly if they wish.

There's also the matter of did Joe legally adopt Barry? If not, then he and Iris wouldn't be siblings in any way in the law's eyes.