If a permanent that isn't a double-faced card becomes a copy of Biolume Egg and then is sacrificed, that card can't be returned to the battlefield transformed. It will remain in the graveyard.

You can't sacrifice Biolume Egg unless another effect allows you to.

You may sacrifice any two lands with the subtype Island to activate Biolume Serpent's ability, even if they aren't basic or aren't named Island.