What would be a good alternative to "in-law" in the event the couple never married, possibly didn't stay together, but a child resulted?

ObscuredBeyond
For example, your relative had a one night stand that led to a baby, and kept the baby. But has almost nothing to do with the child's other parent anymore.

Do you still just use the term "in-law," or would there be a more appropriate way to say it?

I know in Spanish, terms like "yerno" apply due to biology determining the relation, not state declarations. Alas, the English language didn't learn this important lesson from Spanish soon enough. So legal status defines everything in English.

(Which is also why a tale like Oliver Twist would've made very little sense if set in Spain, whereas it made perfect sense in Dickensian England. Spaniards looked at the parents of bastard children like: "You big dummy!" In England, due to the logic of state declaration deciding everything, the child was the one that paid the biggest price, due to their existence breaking the language. In Spain, bastard children only jeopardized the local economy to a much smaller scale and extent, so it was the parents who were scorned, not so much the children. With France, it was 50/50.)

A term like "daughter-out-of-law," or "son's sin partner," while accurate, come off as extremely contempt-laden, as if to imply they are to stand forever condemned for it. But what better terminology is there in English?

Doing some outlining of family trees for a sci-fi. And one guy's DNA was stolen from him by aliens to make a half-clone baby. An alien scientist volunteer is then impregnated through this process, and produces a child a year after the original baby was born. Many years later, the man discovers he has a son with aliens that's almost the same age as himself.

But then, the entire family tree for both his families has to get mapped. And this quickly becomes a nightmare of questionable terminology.
What would be a good alternative to "in-law" in the event the couple never married, possibly didn't stay together, but a child resulted?
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