For me, it was the fact that they used her brother's sperm that puts me off. It feels like its not Arisa's child but her brother's. would have been better if they adopted one or something. Then I would have been all for it.
So what other people have said, but also, then we wouldn't have the hilariously uncomfortable situation of Arisa asking him for it.
Because you know she gave him no warning. Just "Hey bro would you jerk off in this container for me? I wanna get Mari pregnant!" and then her staring expectantly until he kicks her out of the room.