My point was, if you do not eliminate the brothers and sisters, then there are two possible answers: son or nephew--as you found out when you gave "his son" as the answer and were challenged. By eliminating the man's siblings, then there can only be the one answer you gave.
Okay I will put the context for this sentence.. There was a girl of age 16. She was having the signs of pregnancy. Her father was travelling for work, and when he came back. he started to ask his daughter who is the father who is the father who did that to you. And at that very moment her boyfriend enters the house and say "I AM THE FATHER OF HER SON"
The noun fils is masculine, and that is what determines the gender of the possessive adjective. son fils could mean his or her son—which depends on the context, and can make for some weird translations into English when that context isn't given, as smeardink's comment above illustrates.
I retract my previous reply. I've learned to be suspect of Google Translate, and upon further research into your question, I believe you may be correct. All other references to «être» vs «suivre» indicate that context is the only way to distinguish between which word is being used (in the first person present indicative). I also read that there may be a difference in colloquial pronounciation. I would suspect to avoid confusion. So, I suppose in writing it's up to the author to make sure to clarify when context does not. Again, great question!
So what if the child is the product of an extramarital affair? You do not have to be married to someone to have a child with him or her.
Actor Eddie Murphy has a child with Spice Girl Mel B but they have never been married. Eddie could very well say "I am the father of her daughter" or "I am her daughter's father."
More than non-married parents, you also have parents of same gender as possible solutions. The goal of the app is to teach you vocabulary, grammar, and all aspects of the language, not merely conversational French phrases. There are other apps that fulfill this goal if that's what you would like.
his (as in his house, his car, not the phone is his) and her both translate to son or sa or ses. English and French approach this completely differently so there isn't a 1:1 translation.
In English it's all about whether the person doing the possessing is male (his) or female (her), there's no consideration for whether the thing possessed is male or female.
In French it's all about whether the thing (or things) possessed is male or female, they don't care about the person doing the possession.
If there is more than one thing (or person) possessed then they always use ses.
If the thing (or person) possessed is grammatically masculine then they use son.
If the thing (or person) possessed is grammatically feminine but starts with a vowel sound then they also use son because sa followed by a vowel sound is harder to say (compare with using a vs an in English).
If the thing (or person) possessed is grammatically feminine and starts with a consonant sound then they use sa.
To expand on this: When conversing in French, one generally know the context (people and places involved) in which one speaks. So, while at first it might seem vague and confusing to an English speaker, it’s not actually quite different from distinguishing between two different women when saying “her” or two different men when saying “his”. Plus, I’d hate to have to deal with even more word forms to account for both the gender of the noun and the gender of the adjective modifying the noun. A good page to read about the special forms of adjectives to which BenYoung84 refers can be found here: https://www.thoughtco.com/french-adjectives-with-special-forms-1364547