Alpha and omega siblings can become pairs??

Really matters how the author writes the omegaverse. In some bl manga it is possible. In others it seems like family is immune

omegaverse is a made up thing so, like FanGirlQueen99 said, is rlly up to the author how they want to build the world and what rule does it follows. So getting pregnant, marking, heat cycle and all those stuff are all different depending on the author. but most of them would follow the "general rule" to the omegaverse theme.