"My daughter's husband is my son-in-law."
Translation:Le mari de ma fille est mon gendre.
From Latin, but I have not found the history of this word in more recent times.
"Un beau-fils" is used for a step-son, but very rarely for "son-in-law", whereas "une belle-fille" can be either a step-daughter or a daughter-in-law, and very rarely "une bru" can refer to a daughter-in-law.
- please don't use "un beau-fils" if he is your daughter's husband, but "un gendre"
- please don't use "une bru" if she is your son's wife, but "une belle-fille".
I always value your remarks, Sitesurf, but there may be some nuance I'm missing. Collins-Robert gives son-in-law as 1) gendre 2) beau-fils, and elsewhere gives stepson as beau-fils. Collins Reverso online gives the same precedence to gendre, but still lists beau-fils as second option for son-in-law.
Clearly, there appears to be a difference between dictionary French and French as it is commonly used. Or maybe I need a more detailed dictiomnary.
What would be wrong about calling your son-in-law your beau-fils?
Using "beau-fils" instead of "gendre" may be some regionalism, as I have never heard it.
It would not be wrong to use it, since "belle-fille" is daughter-in-law and you would be understood, maybe with a bit of frowning, though. I'm pretty sure your French counterpart would try to check if this man is your partner's son or your daughter's husband.