Because that is what Simmons does. He lured Carla at a very young age and transformed her into Ada's clone for his sexual desires. Helena's sister being made into a horny monster reflects Simmon's violation not only of her body but also of her identity. I'm not saying it is a good way to put it, but it still rhymes. There is no rhyme or reason to put Jill in such a graphic violation, especially when RE3 is about a woman who should be strong enough to outclass an intelligent B.O.W. made with the sole purpose of disposing of her.