Gnome's Things

So I've read this manga (just now actually), it's about a 3some relationship in the world of omegaverse. I've read many 3some and omegaverse mangas (separately) but this is the first manga with both genres in one, however that's not the point, the point is what if the uke get pregnant with a twin but the twin is stepbro/sis. In case you don't understand what I meant, you know how twins are born bc the sperm splits. In this case, the sperm didn't split but there are two sperm from two people (the semes). Does that work? I'm curious

P/s: I don't know much about how kids get deliver to this world nor do I know much about "if u know u know" so this will sound weird and dumb