S.H.E.L.T.E.R. has a threesome with brother (MC)/sister/sister's female friend from the army. In this threesome, brother and sister don't do stuff with each other. On the other hand, the game has MC impregnating his mom and both sisters as part of their efforts to repopulate the world. So if you don't want any actual incest at all, this isnt what you are after.
