Well, as far as I'm aware mules/ligers and other hybrids can be born fertile, they are simply far less likely to be born fertile, due to the mismatched chromosomes. So It's possible that Jill could have had demon/human half-breed in her ancestry that just so happened to be fertile, or perhaps even was male, and for whatever reason the issues of fertility in demon/human half-breeds is only evident in females.Funny how everyone immediately thought about mules. But one problem with that theory is Jill, who has the same abilities to sniff lights but can very much get pregnant. The game either implies or outright tells you she's of demonic descent, which Issyr mentions isn't unheard of but is thought of as largely pointless due to their inability to use lights.
Honestly, though, why not just brown light to change her into someone who can get pregnant? It's sort of hinted at when you help her with her experiments and she sees what you did to Jane and finds out she's pregnant, and she smiles because she thinks it'll work. And I mean, it should, because pregnancies while they're bimbofied carry over when the light wears off, and brown lights can be used to change people into non-humans.
I guess what I'm saying is, we need to go full Zeus mode and make some weird hybrids.
But I do agree that the answer to all of this should really be new and more creative usage of the lights, which does seem to be the way things are going. It seems like as we get stronger, we keep finding more ways to use the lights, beyond what any of the coven would normally be able to use, and even beyond what Delia would be able to use them to do. So I do hope (and think) that we'll be able to impregnate Chang eventually, through whatever creative light use we need to use to make that happen, as well as any other (seemingly) un-impregnate-able women.