Yeah, I'm totally overthinking this, but since the yukkuri in back (who I'd assume is the father) is saying "ze," and the yukkuri giving birth has a braid, are kotatsumuri Marisas an exception to the yukkuris-can't/don't-mate-with-their-own-kind rule?
Rrrrrrrrrule? *imitating kneesocks' accent* Such rrrrrules don't even matter anymore it seems, as now there are these kotatsu marisas suddenly being popular appearing now, which are breaking the rrrrrule that marisas must wear her trademark hat lest she won't be able to take it easy. Yukkuri rrrrrules seem very open to each artists' interpretation and are often ignored, I've yet to see an absolutely concise rrrrrule guideline on yukkuris that everybody follows, and probably won't, since it would be hard to decide which rrrrrules are "official" when dealing with something as generally loose fanworks as yukkuris.
It's not the first time someone makes hybrids or mutations, usually they work outside of the average fanon rules, and rarely last long or have a impact in the "fanon rules/fanon storylines"