They have a mismatched number of chromosomes from their father/mother.

This is already the situation with mules: https://en.wikipedia.org/wiki/Mule#Fertility

Mules and hinnies (female mules) have 63 chromosomes, a mixture of the horse's 64 and the donkey's 62. The different structure and number usually prevents the chromosomes from pairing up properly and creating successful embryos, rendering most mules infertile.

Humans have 23 pairs of chromosomes, so your cambion inherits 23 chromosomes from its mother. Give the demon a very different number of chromosome pairs...say 66. Now your cambion has 89 chromosomes; many of which have no proper pairing.

I don't know if we have any mathematical models for predicting the chances of two cambions successfully reproducing given this situation, but if 1 chromosome difference is enough for mule offspring to be so rare, it seems like only a tiny bit of hand-waving to call it impossible for a cambion with a much larger difference between their paternal and maternal chromosome sets. (Most of the hand-waving would probably be that a cambion would be viable in the first place.)

Note: as it currently stands, this answer does not meet all the OP's requirements.

(The OP's question requires that a cambion be able to produce offspring when mating with a demon. I had missed this requirement when I composed this answer, but it was brought to my attention in comments. I'm waiting to hear back from the OP on a certain point, after which I hope to amend this answer.)

They have a mismatched number of chromosomes from their father/mother.

This is already the situation with mules: https://en.wikipedia.org/wiki/Mule#Fertility

Mules and hinnies (female mules) have 63 chromosomes, a mixture of the horse's 64 and the donkey's 62. The different structure and number usually prevents the chromosomes from pairing up properly and creating successful embryos, rendering most mules infertile.

Humans have 23 pairs of chromosomes, so your cambion inherits 23 chromosomes from its mother. Give the demon a very different number of chromosome pairs...say 66. Now your cambion has 89 chromosomes; many of which have no proper pairing.

I don't know if we have any mathematical models for predicting the chances of two cambions successfully reproducing given this situation, but if 1 chromosome difference is enough for mule offspring to be so rare, it seems like only a tiny bit of hand-waving to call it impossible for a cambion with a much larger difference between their paternal and maternal chromosome sets. (Most of the hand-waving would probably be that a cambion would be viable in the first place.)

They have a mismatched number of chromosomes from their father/mother.

This is already the situation with mules: https://en.wikipedia.org/wiki/Mule#Fertility

Mules and hinnies (female mules) have 63 chromosomes, a mixture of the horse's 64 and the donkey's 62. The different structure and number usually prevents the chromosomes from pairing up properly and creating successful embryos, rendering most mules infertile.

Humans have 23 pairs of chromosomes, so your cambion inherits 23 chromosomes from its mother. Give the demon a very different number of chromosome pairs...say 66. Now your cambion has 89 chromosomes; many of which have no proper pairing.

I don't know if we have any mathematical models for predicting the chances of two cambions successfully reproducing given this situation, but if 1 chromosome difference is enough for mule offspring to be so rare, it seems like only a tiny bit of hand-waving to call it impossible for a cambion. (Most of the hand-waving would probably be that a cambion would be viable in the first place.)

They have a mismatched number of chromosomes from their father/mother.

This is already the situation with mules: https://en.wikipedia.org/wiki/Mule#Fertility

Mules and hinnies (female mules) have 63 chromosomes, a mixture of the horse's 64 and the donkey's 62. The different structure and number usually prevents the chromosomes from pairing up properly and creating successful embryos, rendering most mules infertile.

Humans have 23 pairs of chromosomes, so your cambion inherits 23 chromosomes from its mother. Give the demon a very different number of chromosome pairs...say 66. Now your cambion has 89 chromosomes; many of which have no proper pairing.

I don't know if we have any mathematical models for predicting the chances of two cambions successfully reproducing given this situation, but if 1 chromosome difference is enough for mule offspring to be so rare, it seems like only a tiny bit of hand-waving to call it impossible for a cambion with a much larger difference between their paternal and maternal chromosome sets. (Most of the hand-waving would probably be that a cambion would be viable in the first place.)

They have a mismatched number of chromosomes from their father/mother.

This is already the situation with mules: https://en.wikipedia.org/wiki/Mule#Fertility

Mules and hinnies (female mules) have 63 chromosomes, a mixture of the horse's 64 and the donkey's 62. The different structure and number usually prevents the chromosomes from pairing up properly and creating successful embryos, rendering most mules infertile.

Humans have 23 pairs of chromosomes, so your cambion inherits 23 chromosomes from its mother. Give the demon an very different number of chromosome pairs...say 66. Now your cambion has 89 chromosomes; many of which have no proper pairing.

I don't know if we have any mathematical models for predicting the chances of a cambion successfully reproducing given this situation, but if 1 chromosome difference is enough for mule offspring to be so rare, it seems like only a tiny bit of hand-waving to call it impossible for a cambion. (Most of the hand-waving would probably be that a cambion would be viable in the first place.)

They have a mismatched number of chromosomes from their father/mother.

This is already the situation with mules: https://en.wikipedia.org/wiki/Mule#Fertility

Mules and hinnies (female mules) have 63 chromosomes, a mixture of the horse's 64 and the donkey's 62. The different structure and number usually prevents the chromosomes from pairing up properly and creating successful embryos, rendering most mules infertile.

Humans have 23 pairs of chromosomes, so your cambion inherits 23 chromosomes from its mother. Give the demon a very different number of chromosome pairs...say 66. Now your cambion has 89 chromosomes; many of which have no proper pairing.

I don't know if we have any mathematical models for predicting the chances of two cambions successfully reproducing given this situation, but if 1 chromosome difference is enough for mule offspring to be so rare, it seems like only a tiny bit of hand-waving to call it impossible for a cambion. (Most of the hand-waving would probably be that a cambion would be viable in the first place.)