Ambiguità con il genitivo sassone

Hello guys,
mentre ero assorto tra vari pensieri mi sono venuti alla mente alcuni ricordi di quando studiavo grammatica inglese, in particolare su un esempio per il genitivo sassone.
La frase: "I genitori di Carl e Mary sono in casa" viene tradotta:
"Carl's and Mary's parents are at home" se Carl e Mary non sono fratelli, oppure come
"Carl and Mary's parents are at home" se Carl e Mary sono fratelli.
Quest'ultima frase mi crea un po' di confusione e mi sembra ambigua in quanto potrebbe essere la traduzione anche della frase
"Carl e i genitori di Mary sono in casa".
Mi sono perso qualcosa o è un'ambiguità da cui non si scappa?

Thank you for help.

There isn't any language lacking ambiguity

There isn't any language lacking ambiguity

I couldn't agree more.
OT/
Have you taken the FCE exam yet ? I passed it last month!! Mind you, now I'm studying for the CAE. I've already bought some books to improve my English grammar and vocabulary level furtherly. As you might expect the writing part is probably the hardest. Not to mention the speaking skills required. Fluency and a wide range of vocabulary are requirements you have to meet if you want to reach an advanced level.
How about your listening skills ?

Hi mister k
I am glad you pass first, congratulations , I'm gonna do it this summer (August is the best choice), I have had a lot of work until now

About listening, I love watching tv series as you should know, but the best are American and my teacher discourage me to see those (British people think Americans are less educated, less cultiveted... more ignorant, you know) they are obiouvsly wrong, that's just envy
Anyway she has told me that the listening is more of British English, so now I am watching "Sherlock", Have you ever seen it?
I reach the third season

Thank you Gio, I'm proud of passing my exam.
Yes, of course, I've seen the series 'Sherlock', not every episode, just a few of them. I liked it. I think the best thing you can do is to exercise with FCE listening papers. You might find them a bit boring but it's a really useful way to put into practice your listening abilities. You'd help yourself a great deal that way. No chance of failing the exam.

Hi MisterK
Actually there is another one series I loved, about jail: "Oz".
I have seen all episodes.
Now I am doing my boring homework, I use a book with listeng part that my teacher suggested.

What book are you studying on ? You can find tons of material on youtube for the listening part.
I've never seen Oz. Apart from Prison Break and Dexter I'm not a tv series lover I must confess. As a rule I have much more fun watching native English speaker talking on Youtube. Actors, singers, journalists, politicians, for istance.

ever seen letterman show?

Of course, it's one of the shows I like most. It teaches a lot and it's highly entertaining. What do you think of it ?

I think mr letterman has got a lot of humor and (auto)irony, goods rare nowdays.