[Airandou] Imouto Haramasenai to Level Up Dekinai Isekai Tensei

Yeah it's an isekai. Brother and sister get summoned to another world. Her class is "Knight" and basically to be the hero and his power is to level her up by impregnating her. Each time he fertilizes his sister she gains a level and her body absorbs the fertilized egg to recover her mana since being a pregnant adventurer would be a problem. To get to max level he needs to fertilize 99 of her eggs. Even though it says that his skill is to impregnate her, obviously she won't be impregnated because impregnation is when an egg is implanted and begins to grow into a fetus. Since her body absorbs the fertilized egg for mana, that's not going to happen.

I wanna see a sequel where they return to the normal world, and he still knocks her up

Is this an Isekai?