Translation:This letter is related to my mother.
Not necessarily, "is related to" could be used to imply it's about a subject which concerns your mother but is not your mother directly. E.g. "I noticed you got a letter from that Alzheimer's charity this morning. What's that about?" "Oh it's related to my mother and her recent fundraising efforts."
Actually we wouldn't say "Oh it's related to my mother and her recent fundraising efforts." We would say "Oh it's about my mother and her recent fundraising efforts." This makes more sense in English. "Is related to" does imply a family connection, because English people associate this with family.
When I said "we" I was referring to English people in general. I am English, yet that doesn't mean to say that everyone on here learning French is also English. Therefore, I wasn't aware that you are also English. I wasn't intending to offend you in any way. The example you posted doesn't make sense to me, so I was just explaining that generally people would say it in a different way. I'm studying AS English language, so I guess I think about the way in which language is used quite a lot. I'm sorry if my comment offended you; it wasn't intentional. I had no idea that you are also English. If I had known I would have explained my point in a better way. I hope you can accept my apology.
Don't worry, I wasn't really offended - I was looking for the most tactful way to point out that one shouldn't assume one's own experience and therefore view of even one's native tongue is identical to other native speakers. Something I've repeatedly noticed on Duolingo is that idiolects can be surprisingly varied when it comes to nuances of translation. Moreover, when bleached of their context, otherwise acceptable sentences can suddenly sound disjointedly unnatural. I recommend reading this:
to give you some idea of the imagination that can provide a seemingly impossible sentence sufficient context to become meaningful. Especially "Attempts at meaningful interpretations".
The above example shows that it worked better when you added "and her recent fundraising efforts." Without something that it is related to and just a person, it would have created the same confusion. People are related to each other, because they are each other's relations. I am from California. Are you from England? Here in the USA, we would more likely say "The letter is about my mother." and it is also an accepted answer.
I'm pretty sure you can't vouch for what every single English speaker across a number of regions / countries is likely to say.
"relative to" is gramatically correct, and is in fact said by English speakers.
It's also tricky when making assertions about what people say in "every day" speech. I'm sure that word choice varies quite significantly across different regions, countries, and socio-demographics, for example.
Unless the letter referred solely and specifically to my mother, I wouldn't say "is about".
A letter from a doctor describing her medical condition is about her.
A newsletter from an organisation of which she is a member is not.
A letter from a friend describing a recent holiday isn't about her, either.
For formal use another way to reference somebody or something is to use 'regard'. French cognate yay!
e.g This letter is with regard to my mother... (it concerns my mother, is referring to my mother, my mother is the subject)
Note that emails in english have 'RE:' before the subject if you reply to them, this is short for 'Regarding: subject'
From: Hopethishelps@qqch.com Subject: RE: My Mother